When running the hybrid configuration wizard on an Exchange 2016 server (in an Exchange 2010 to Exchange 2016 migration) to create a (classic) hybrid configuration, the wizard failed with several error messages as shown in the following screenshot:
Or in plaint text:
Microsoft.Exchange.Migration.MigrationServerConnectionFailedException. The connection to the server ‘mail.contoso.com’ could not be completed.
Microsoft.Exchange.MailboxReplicationService.MRSRemoteTransientException The call to ‘https://mail.contoso.com/EWS/mrsproxy.svc’ failed. Error details: The HTTP request was forbidden with client authentication scheme ‘Negotiate’. –> The remote server returned an error: (403) Forbidden..
Microsoft.Exchange.MailboxReplicationService.MRSRemotePermanentException The HTTP request was forbidden with client authentication scheme ‘Negotiate’.
Microsoft.Exchange.MailboxReplicationService.MRSRemotePermanentException. The remote server returned an error: (403) Forbidden.
I did the following troubleshooting steps:
Manually creating a migration endpoint using the Exchange Online Admin console. This failed as well.
Firewall restrictions were in place, but the Microsoft IP ranges were all configured correctly
Checked the MRS. It was enabled (well, that’s wat the Exchange PowerShell said) and authentication was set correctly
Eventually it turned out that the MRS was the problem. Although Exchange Management Shell returned that the MRS Proxy was enabled, it was not functioning correctly. I disabled the MRS proxy, enabled it again and restarted IIS using the following PowerShell commands:
I am working on an Exchange 2010 project (no typo!) where all mailboxes need to be moved to Exchange Online. Since Exchange 2010 is out-of-support for years, creating an Exchange 2010 hybrid environment is not a good idea. So, instead I installed Exchange 2016 and created a hybrid Exchange 2016 environment. But that brings it challenges as well.
The first issue we ran into when creating the hybrid configuration was the TLS 1.2 issue. Exchange 2016 (on Windows 2016) supports TLS 1.2, but it needs to be enabled. To enable TLS 1.2 on Exchange 2016, follow the instructions on the Exchange Server TLS configuration best practices article on the Microsoft website.
When running the Hybrid Configuration Wizard for the first time you must add the domains for federation with Office 365. DNS TXT records were added for all four domains but validating failed after validating the first domain. The wizard hangs on validating subsequent domains.
Removing the other three domains from the wizard and thus validating with only one domain solved our problem and the wizard successfully finished. Later on we were able to add the other domains, but only one at a time.
My previous blogpost was about DANE and discussed how DANE can be used to make TLS negotiations between mailservers more secure. Another topic in this area is MTA-STS. MTA-STS stands for Message Transfer Server – Strict Transport Security and MTA-STS is a mechanism to enforce the use of TLS and the use of a valid 3rd party server certificate.
MTA-STS, DNS and Policies
When an MTA-STS capable servers wants to send an email, it first retrieves the MX record for the recipient domain. The second step is that the sending server check for an MTA-STS record in DNS. This record looks like:
_mta-sts.exchangelabs.nl. IN TXT "v=STSv1; id=202306242147;"
The id is an identifier and defines the version of the MTA-STS record when changes are made to the MTA-STS record. A good practice is to create an identifier based on the date and time of the last change. In this example, it is June 24, 2023 at 9:47pm.
The next step is that the sending servers looks for a policy. This policy is not stored in DNS, but on a website. The URL for this policy looks like: https://mta-sts.exchangelabs.nl/.well-known/mta-sts.txt. The subdomain mta-sts, the filename mta-sts.txt and the directory .well-known (including the dot) directory are mandatory for the MTA-STS policy. It must also be secured using a valid 3rd party server certificate.
Version identifies the version of MTA-STS but must always be STSv1 (for now at least).
Mode defines how the policy must be applied:
Enforce: Sending MTAs MUST NOT deliver the message to hosts that fail MX matching or certificate validation or that do not support STARTTLS.
Testing: Sending MTAs that also implement the TLSRPT (TLS Reporting) specification [RFC8460] send a report indicating policy application failures (as long as TLSRPT is also implemented by the recipient domain); in any case, messages maybe delivered as though there were no MTA-STS validation failure.
None: In this mode, Sending MTAs should treat the Policy Domain as though it does not have any active policy.
MX defines all MX records in use by the recipient domain. This can be one entry, but it can hold multiple MX records, each on a separate line as shown in the policy above.
Max_age defines the time (in seconds) that the MTA-STS policy can be cached by a mail server. In this example, the policy is cached for 604800 seconds, which equals to 1 week. When a sending server must send a new email within a week, the policy is still cached. After checking the MX record the server retrieves the TXT record from DNS (as explained in the second step above) and when the identifier has not changed it uses the policy that is cached. If the identifies has changed within the lifetime of the cached policy, a new policy is downloaded.
So, in my example an MTA-STS capable mail server will check the MTA-STS policy and only connects to my mail server using TLS 1.2 (this is enforced with MTA-STS when mode is set to ‘enforce’) and only when a certificate that matches the FQDN is used. When authentication fails for an entry, the sending server continues with the next line in the policy, in my example with the MX record pointing to Exchange Online.
An interesting option in MTA-STS is reporting. DMARC has a reporting function as well, but reports are only sent by receiving domains. Reporting in MTA-STS is performed daily by sending mail servers that supports MTA-STS and TLS-RPT.
To configure the reporting functionality, create a mailbox in Exchange 2019 or Exchange Online and assign it an email address like TLSReports@Exchangelabs.nl. The next step is to configure the following DNS TXT record:
_smtp._tls.exchangelabs.nl. 3600 IN TXT v=TLSRPTv1;rua=mailto:TLSReports@exchangelabs.nl
There are several online tools available for checking the MTS-STS record. Just like in my other blogs, I often use MXToolbox to check for DNS records as shown in the following screenshot:
The first one is obvious, the second line is a Microsoft IP address, the third line is a Yahoo IP address. So, Google, Microsoft and Yahoo are using MTA-STS when sending email.
MTA-STS versus DANE
MTA-STS and DANE share a common concept, that is to secure the (initial) communication between mail servers. The ‘problem’ with DANE is that is relies on DNSSEC and the global roll-out of DNSSEC is very slow (to put it mildly).
MTA-STS was developed to overcome the slow roll-out of DNSSEC (since it does not use DNSSEC of course). MTA-STS can be seen as a ‘light-weight’ version of DANE and it will be sufficient for most customers.
And how about Exchange?
Just like with DANE, the ugly part is that Exchange 2019 does not support MTA-STS. You can configure the MTA-STS record in DNS and the policy on a website so that MTA-STS capable servers use your Exchange 2019 server safely. But for sent messages by Exchange 2019, MTA-STS is a no-go, it does not support it and most likely will never do.
Exchange Online on the other hand does support MTA-STS (since the beginning of 2022) for both inbound and outbound messages. The only thing you must do to enable it for inbound messages is create the TXT record in DNS and create and publish the MTA-STS policy.
In the past I have written about SPF, DKIM and DMARC for email authentication in Exchange server to improve e-mail security. Additional topics to improve security are secure DNS (DNSSEC) and DNS-based Authentication of Named Entities (DANE). Where SPF, DKIM and DMARC are focusing more on the email messages and the sending hosts they are coming from, is DANE more focusing on setting up the TLS connection between mail servers. DANE has a dependency on DNSSEC but since my focus is on Exchange and Exchange Online, I don’t discuss DNSSEC and leave that to my provider (DANE only works when DNSSEC is enabled).
When two mailservers setup a connection, they negotiate a common TLS protocol and this handshake as it is called is unencrypted. As such, this handshake is vulnerable for man-in-the-middle or downgrade attacks.
DANE is using a special DNS record (a TLSA record, Transport Layer Security Authentication) where an organization can publish information regarding the certificate on the mail server and thus the TLS options available.
The TLSA record for my own domain is shown in the following example:
_25._tcp.smtphost.exchangelabs.nl IN TLSA 3 1 1 50167c478a2f536a88ee9ec232b14b0d223c2d5bdc837451eee104c153376cbe
When a mailserver wants to send email to my Exchange environment it retrieves the MX record first which is smtphost.exchangelabs.nl (this is an Exchange 2019 Edge transport server, with a Digicert certificate that matches the servername). When DNSSEC is not used at the recipient domain, opportunistic TLS is used. When DNSSEC is used at the recipient domain, a DANE capable mail server will check for a TLSA record.
When a TLSA record is found and retrieved, the sending server sets up a TLS connection with the receiving server, which in turn returns the fingerprint of the certificate. The sending server compares the fingerprint with the information found in DNS and when it matches, the connection is established and communication between the two servers continue, and the email is sent.
So, how do you create the TLSA record? There are publicly available webservers that can create TLSA records. Shumon Huque is a software engineer and technologist based in Washington DC who has a site that can generate TLSA records. As a bonus his site can also check TLSA records.
To generate a new TLSA record, navigate to his website on https://www.huque.com/bin/gen_tlsa and fill out the necessary information as shown in the following screenshot:
The easiest way to create a PEM format X.509 certificate is using the MMC certificate snap-in. Select your certificate and export it. Select the ‘No, do not export the private key’ option, select the ‘Base-64 encoded X.509 (CER)’ option and export the certificate. You can use Notepad to open the certificate export, and copy-and-past this into the PEM format X.509 textbox.
Select the port number (25), the protocol (TCP, not SMTP) and enter the domain name. This is the FQDN of the receiving server, so in my example it is smtphost.exchangelabs.nl and click Generate.
It will show the generated TLSA record, including the certificate information as shown in the following screenshot:
For my domain the following TLSA record is generated:
_25._tcp.smtphost.exchangelabs.nl. IN TLSA 3 1 1 a63d74fc7ec1acad702017d13479a1b60b36f234ccb96b90f97c9619ba2c91ab
How is this TLSA record structured?
TCP port 25 and the FQDN of the server make up the first part of this TLSA record.
The first number after the TLSA text is the certificate verification or the certificate usage on the server and this number can have 4 values:
0 – CA Constraint. The certificate provided when establishing the TLS connection must be issued by the listed root-CA or one of its intermediate Certificate Authorities.
1 – Service Certificate constraint. – The certificate used must match the TLSA record, and it must also pass the certification path validation to a trusted root-CA.
2 – Trust Anchor Assertion. The TLSA record must match the certificate of the root CA, or one of the intermediate CAs of the certificate in use by the mailserver.
3 – Domain Issued Certificate. The TLSA record matches the used certificate itself. The certificate does not need to be signed by other parties and as such a self-signed certificate can be used (this is interesting on an Exchange server).
In my TLSA record a value of 3 is used for the certificate usage, so the TLSA record must match the certificate on the Exchange 2019 server.
The second number that is used is the selector. The selector can have two values:
0 – The entire certificate is used for matching.
1 – Only the public key of the certificate is used for matching.
In my TLSA record the value is 1 for the selector, so only the public key is used.
The third number that is used is the matching type. The matching type can have three values:
0 – the entire information select is present in the certificate associated data (the last text string in the TLSA record)
1 – The SHA-256 hash of the public key of the certificate must match the certificate associated data.
2 – The SHA-512 hash of the public key of the certificate must match the certificate associated data.
In my TLSA record a value of 1 is used for matching, to a SHA-256 has of the public key of the Exchange server certificate must match the data in the TLSA record.
The next step is to add the generated TLSA record in DNS and check the TLSA record once added.
You can use the same site (https://www.huque.com/bin/danecheck-smtp) to check the TLSA record. The site queries the MX record for your domain and checks the accompanying TLSA record. The result is shown in the following screenshot:
In this blog I am discussing DANE for mail server, but it can also be used for other services. For a website a TLSA record can be:
_443._tcp.www.exchangelabs.nl. IN TLSA 3 1 1 a63d74fc7ec1acad702017d13479a1b60b36f234ccb96b90f97c9619ba2c91ab
Or for IMAP it can be:
_995._tcp.mail.exchangelabs.nl. IN TLSA 3 1 1 a63d74fc7ec1acad702017d13479a1b60b36f234ccb96b90f97c9619ba2c91ab
The ugly part is that Exchange 2019 does not support DANE out of the box. For incoming mail, you can configure the TLSA record based on the certificate that is installed on the Exchange (Edge Transport) server. A DANE capable mail server automatically checks for a TLSA record and when available, use this to setup the TLS connection.
Exchange Online on the other hand only supports DANE for outbound connections at the time of writing. As an Exchange Online customer, there’s nothing you must configure, it’s available for everyone. DANE for inbound mail will become available in the future, but when is unclear. According to Microsoft it is ‘near future’ but unfortunately this is already the case for quite some time.
One final (and very important) closing note: When you have configured a TLSA record for your (inbound) Exchange server you MUST update your TLSA record when renewing the Exchange certificate. You must also do this when the Exchange server is used in a hybrid configuration, since Exchange Online will check for a TLSA record. Mail flow from Exchange Online to Exchange 2019 will halt and the following error message is shown in Exchange Online message trace:
Reason: [{LED=450 4.7.323 tlsa-invalid: The domain failed DANE validation [Message=450 4.7.323 tlsa-invalid: The domain failed DANE validation] [LastAttemptedServerName=smtphost.exchangelabs.nl] [LastAttemptedIP=185.116.41.45:25] [SmtpSecurity=11;-1] [HE1EUR01FT069.eop-EUR01.prod.protection.outlook.com 2023-06-06T10:23:06.0. OutboundProxyTargetIP: 185.116.41.45. OutboundProxyTargetHostName: smtphost.exchangelabs.nl
This is solved when a new TLSA record is generated and published in DNS.
On June 13, 2023 Microsoft has released Security Updates for:
Exchange 2019 CU13
Exchange 2019 CU12
Exchange 2016 CU23
There are no Security Updates released for older versions of Exchange 2016 and Exchange 2019, these are the only supported versions. There are also no Security Updates for Exchange 2013 since this is completely out-of-support. If you are still running on Exchange 2013 you must seriously consider upgrading to Exchange 2019 or Exchange Online.
The following vulnerabilities are addressed with these Security Updates:
When possible, try to run the latest Cumulative Update for Exchange 2016 or Exchange 2019.
Exchange Security Updates are cumulative, so a Security Update contains all fixes that were released in earlier Security Updates (for a specific Exchange Cumulative Update).
Exchange Security Updates are specific for an Exchange Cumulative Update, so you cannot install an Exchange Security Update for Exchange 2019 CU13 on an Exchange 2019 CU12 server.
Security Updates must be installed on hybrid servers as well, even if there are no mailboxes anymore on these hybrid servers.
If you have a management server with only the Exchange server management tools installed, you must install Security Updates as well.
Of course, test Security Updates in a test environment first.
Jaap Wesselius 
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