So, today I found out that outbound mail from my jaapwesselius.com did not have a DKIM signature (after mail was blocked by prodigy.net). I have my jaapwesselius.com running on WordPress.com. To do this, WordPress requires to have DNS hosted with them. No problem, but adding a DKIM record in WordPress DNS is not possible, it fails with a TXT records may not exceed 255 characters error message as shown below:
The solution is relatively simple. You can add a CNAME record for the original DKIM record. For example, have safemail._domainkey.jaapwesselius.com point to something like safemailhop.exchangelabs.nl (I own that domain too, and DNS is hosted at my provider Argeweb).
In the previous two blog posts I’ve explained how to implement Exchange Online Protection as a message hygiene solution for your on-premises Exchange environment, both for inbound as well as outbound mail flow.
In this blog post I’ll go more into detail when configuring Exchange Online Protection
SPF, DKIM and DMARC
When Exchange Online Protection is fully up-and-running you can continue configuring SPF, DKIM and DMARC for message authentication purposes. This will make sure your email domain is much harder to spoof and receiving email systems know that the source of your email is a trusted source.
When only using Exchange Online Protection your SPF record will look like v=spf1 include:spf.protection.outlook.com -all.
The good thing about Exchange Online Protection is that it supports DKIM signing and verification out of the box. The only thing you have to do is enable it in the Exchange Admin Center!
Logon to the Exchange (Online Protection) Admin Center, select protection in the navigation menu and click DKIM in the toolbar. When you click enable, an error message (in yellow) is shown that you need to create the appropriate DNS records first (click to enlarge).
The error message reads (for search engine purposes):
CNAME record does not exist for this config. Please publish the following two CNAME records first. selector1-domain-com._domainkey.tenantname.onmicrosoft.com
The description isn’t that clear when you’re not a DKIM guru, but you need to create the following CNAME records:
When the DNS records are created you can use the MXToolbox to check if they are valid:
If an email is sent from the on-premises Exchange server via Exchange Online Protection to for example Gmail, you can check the headers. If configured correctly you can see the SPF check passes, you can see the DKIM signature created by EOP and you can see the authentication results as well. It should read spf=pass and dkim=pass under Authentication Results as shown in the following screenshot (click to enlarge):
When DKIM and SPF are configured correctly you can create a DMARC record in public DNS. A DMARC policy will tell a receiving mail server what to do with email that does not comply with other settings. For example, if email is coming from a mail server that’s not listed in the SPF record it might well be spoofed. If the DKIM signature is missing it might be spoofed, if the DKIM signature is not valid, the message might be tempered with. If this is the case, you can define a policy that will reject such a message. The DNS record will be like this:
In this blogpost I showed you how to configure SPF, DKIM and DMARC in Exchange Online Protection to prevent spoofing by malicious users.
In my next blog I’ll go more into detail about configuring the message hygiene options themselves in Exchange Online Protection.
In the previous two blogpost I discussed how to configure SPF, DKIM and DMARC for outbound messages using the TrendMicro Hosted Email Security solution, including Office 365 centralized mail transport. In this blog I’ll discuss SPF, DKIM and DMARC for inbound messages (i.e. the verification part) using the TrendMicro solution.
For inbound protection there’s not too much configuration to do, the only thing when using an online service is enabling the services.
In HEC Console select Inbound Protection and select Domain-based Authentication. Here you’ll find the options for SPF, DKIM and DMARC:
Select the Sender Policy Framework (SPF) option and in the SPF window check the enable SPF checkbox and when needed, check the Insert an X-header into email messages checkbox:
In my previous blog I showed you how I implemented Trend Micro Hosted Email Security (HES) in my Exchange 2010 environment. Interesting case, it’s an Exchange 2010 hybrid environment with mailboxes in on-premises Exchange 2010 as well as mailboxes in Exchange Online. Centralized mail transport is used, so mail to and from Office 365 always routes via HES and the on-premises Exchange 2010 servers to Exchange Online. In this blog I will focus on implementing SPF, DKIM and DMARC in Trend Micro Hosted Email Security.
DKIM is about signing header information using a private key, and to decipher the signature you need a public key which is stored in public DNS, accessible for every mail server on the Internet. No need to worry about the configuration, HES will deliver all the details.
In the HES console select Outbound Protection and select DomainKeys Identified Mail (DKIM) Signing.
A couple of years ago I have been working with the TrendMicro Hosted Email Security (HES) solution and I was very satisfied with it. With the upcoming SPF, DKIM and DMARC awareness I was looking for online solutions that offer this kind of security measures and I found that HES now offers these solutions as well.
I have a hybrid Exchange environment with multi-role Exchange 2010 servers, Exchange 2010 Edge Transport servers and a hybrid configuration. There’s no dedicated Exchange 2016 server for this, the hybrid configuration just uses the existing Exchange 2010 servers. And this works well. There’s an additional namespace o365mail.inframan.nl, this is used solely for SMTP communication between Exchange Online and the on-premises Exchange 2010 servers (without the use of the Edge Transport servers). The configuration looks like this:
This a hybrid configuration with a centralized mailflow. All email is sent and received through the on-premises Exchange environment, including email from and to Office 365. So, email sent to the internet by users in Office 365 are sent first to the Exchange 2010 servers, and then via the Edge Transport servers to the Internet. This way you have full control over your Internet mail flow.
The Edge Transport servers don’t do a great job when it comes to message hygiene. You can configure Realtime Block Lists (RBL) like Spamhaus, configure content filtering using word lists and attachment filtering, but still (a lot of) spam ends-up in the user’s mailboxes. Therefore 3rd party solutions like Cisco Email Security Appliance (ESA, formerly known as IronPort) are used in front of on-premises Exchange solutions