Tag Archives: SMTP

Exchange 2010 hybrid, SMTP, SSL Certificates and Subject Alternative Names

On every Exchange server you need SSL certificates for authentication, validation and encryption purposes. For SMTP you can use the self-signed certificate. Exchange 2010 uses opportunistic TLS, so the self-signed certificate will do in this scenario. If you need to configure domain security (mutual TLS) on Exchange, you need a proper 3rd party SSL certificate for this.

SMTP communication between Office 365 and Exchange in a hybrid scenario is an example of mutual TLS or domain security. A proper 3rd party SSL certificate is needed on your Exchange server.

I was always under the impression that mutual TLS can only use the Common Name of the certificate, which in my scenario is CN=webmail.inframan.nl. After a previous blogpost there was an interesting discussion (see the comments of this particular blogpost) about this, so now it’s time to do some testing.

Originally I had a Digicert SSL certificate with Common Name CN=webmail.inframan.nl, and a Subject Alternative Name entry autodiscover.webmail.com. During the HCW I entered webmail.inframan.nl and selected the proper certificate.

It was time to renew my SSL certificate, so I added an additional SAN entry o365mail.inframan.nl.


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Azure AD Connect Unable to update this object

In earlier blog post I explained how to create user account on-premises and accompanying Mailboxes in Office 365. This is possible with or without an Exchange server on-premises. The latter works, but it’s not supported.

There are also scenarios where you have cloud identities in Office 365 that you want to connect to user accounts in an on-premises Active Directory, so basically converting the cloud identity to a synced identity. This is a common scenario for example when moving from one tenant in Office 365 to another tenant, of maybe when moving from Groupwise or Notes to Office 365.

Suppose we have a cloud identity in Office 365 for a user named Chong Kim, he has an E3 license, a username ckim@exchangelabs.nl and this is also his primary SMTP address.

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Change SMTP mail flow in hybrid scenario

After building a hybrid Exchange environment as outlined in a couple of previous blog posts we have an Exchange 2013/2016 environment where some Mailboxes exist on-premises and some Mailboxes exist in Exchange Online. Autodiscover is still pointing to the on-premises environment, and so are the MX records. Inbound SMTP mail flow from the Internet is still accessing the on-premises Exchange 2016 Edge Transport servers before being delivered to the intended recipients.


Figure 1. The Exchange hybrid environment with Mailboxes on-premises and in Exchange online.

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Upgrade Hybrid Server to Exchange 2016

I’m running a coexistence scenario with Exchange 2013 and Exchange 2016 without too many issues. My hybrid server is running on Exchange 2013 from the beginning, and it is time to upgrade this server to Exchange 2016.

If you have configured your Exchange environment correctly the hybrid server is nothing special. In my environment the hybrid server is just used for sending SMTP messages between Exchange Online and Exchange on-premises, and it is used for migrating Mailboxes back and forth.

Upgrading the existing Exchange 2013 hybrid server to Exchange 2016 is actually just a matter of installing a new Exchange 2016 Mailbox server, configure it correctly like the old Exchange 2013 hybrid server and rerun the Hybrid Configuration Wizard application.


Figure 1. The new hybrid server (hybrid02) will be installed next to the old hybrid server (hybrid01)

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Maximum number of concurrent connections has exceeded a limit

Customer is running a web application and this web application is able to send SMTP messages, for example after a new user registration or a ‘forgot my password’ option that sends out a link for resetting a password.

This application is not always able to send out messages, when it’s not able to the following error is logged on the web server:

Service not available, closing transmission channel. The server response was: 4.3.2 The maximum number of concurrent connections has exceeded a limit, closing transmission channel.

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