Why emails with no sender (e.g. automatic replies) are excluded from processing by CodeTwo

The email routing mechanism in CodeTwo Email Signatures 365 uses an Exchange Online transport rule (mail flow rule) to identify emails to be processed by CodeTwo and get cloud (server-side) signatures. Since the initial release of our cloud service, the exceptions set in the rule are intended to avoid loops and duplicate signatures, and to exclude specific emails from being routed through CodeTwo – emails that are not sent directly by users, but by an automated process or a bot, including out of office messages and calendar messages (e.g. meeting requests).

With the May 2024 update to the CodeTwo service, we further tightened the rule. It now also excludes emails with unspecified sender data (null sender address), like Microsoft 365 automatic replies or automated emails, from being routed through CodeTwo. This update applies to Microsoft 365 tenants that use CodeTwo in the cloud (server-side) or combo signature mode and:

• registered with CodeTwo after May 21, 2024;
• performed automatic configuration (or reconfiguration) of Exchange Online connectors in CodeTwo Admin Panel after May 21, 2024.

Why this change?

Microsoft is consistently tightening their policies regarding mail flow and routing requirements in Exchange Online (as shown in this Microsoft article), which very often affect emails with null sender (such emails end up being blocked or rejected due to incorrect or missing configuration). CodeTwo fully complies with these requirements if set up correctly, but to future-proof your organization and minimize the risk of any of your emails being blocked (especially if you are generating large numbers of automated emails per day), we decided to tighten up the CodeTwo Exchange transport rule to prevent emails with unspecified sender data from being routed through our service.

Does this change affect my email or signatures in any way?

The new exception added to the CodeTwo Exchange transport rule compliments the existing rule settings. The rule now more effectively identifies and filters out emails with the null sender address. Regular emails (that include the sender data) are not affected by this change.

You will get this update only if you registered a Microsoft 365 tenant with CodeTwo or performed the automatic (re)configuration of Exchange Online connectors after May 21, 2024. If you previously configured the CodeTwo service to add cloud (server-side) signatures to automated emails, but you noticed these emails no longer get signatures, you might want to enable routing such emails through CodeTwo. If so, go to this section for more info.

Note that CodeTwo Outlook (client-side) signatures are not affected by this change in any way.

Do I need to reconfigure Exchange Online connectors for my tenant now?

No immediate action is required on your part. However, we recommend you update the CodeTwo Exchange transport rule in your Microsoft 365 tenant at the earliest opportunity for the same reasons as mentioned in this section. 

To do this you can: 

1. (Recommended*) Manually add the new exception to the CodeTwo Exchange transport rule, as described here.
2. Reconfigure CodeTwo connectors automatically as described here.

How to enable routing emails with null sender through CodeTwo?

Before you proceed, make sure the CodeTwo service is configured correctly in your Microsoft 365 tenant. In particular, verify:

1. The configuration of CodeTwo Exchange Online connectors (inbound and outbound).
2. The configuration of the CodeTwo Exchange transport rule.
3. If the CodeTwo domain is added to your tenant’s accepted domains (see how to find the CodeTwo domain). 
4. If the CodeTwo cloud service domain is added to the SPF record of your tenant (learn more). 

Also, check all email addresses from which you plan to send automatic messages and adjust the frequency of emails they send. Email addresses that cause or have the potential to cause infinite email loops might be blocked.

To enable routing emails with unspecified sender data through CodeTwo, perform the following steps: 

1. Sign in to the Exchange admin center.
2. Navigate to Mail flow > Rules. Select the CodeTwo Exchange transport rule and click Edit rule conditions (Fig. 1.).

Fig. 1. Opening the CodeTwo Exchange transport rule for editing.

3. In the Except if section, delete the following exceptions:
   • The message headers > includes any of these words
   • The sender > address matches any of these text patterns
   and click Save to apply your change (Fig. 2.).

Fig. 2. Removing two exceptions from the CodeTwo Exchange transport rule.

4. Finally, click Done to close the rule editing pane. Wait up to 30 minutes for the changes to propagate.

After you make this change, be sure to monitor your mail flow to check if all emails with null sender reach their recipients. If not, you might want to consider reverting this change by configuring the CodeTwo Exchange transport rule with default settings (learn more). Note that you will need to repeat the same steps each time you perform automatic (re)configuration of Exchange Online connectors in CodeTwo Admin Panel (as long as you want to continue rerouting your automated emails through CodeTwo).

Also, once these exceptions are removed, all messages that are missing the sender's address in the Return-Path field will be processed by CodeTwo. If this becomes a problem, consider a way of working around this issue by configuring the scope of senders in CodeTwo Email Signatures 365, so that it only covers the desired users.