Blocked Azure IP causes problems with email delivery

Problem:

An email sent by a user does not reach the recipient. The sending user might get a non-delivery report with the following error:

550 5.7.1 Service unavailable, Client host [<IP address>] blocked using Customer Block list (AS16012607).

where the <IP address> is an IP address of a certain Azure region. See a complete list of IP addresses of the CodeTwo cloud service regions.

Solution:

The problem occurs when IP Block list in the Microsoft Defender XDR (Microsoft 365 Defender) portal is configured to block an IP address of a CodeTwo cloud service region. Removing the IP from the IP Block list solves the problem.

1. Click this link to access the Anti-spam policies page - use your admin's credentials to sign in.

   Info

   To manually access the page, go to Email & collaboration > Policies & rules > Threat policies > Anti-spam.

2. Select Connection filter policy (Default) to open the settings panel, and click Edit connection filter policy (Fig. 1.).

Fig. 1. Accessing connection filter policy settings.

3. Remove the IP address which matches the address of any CodeTwo service region from the Always block messages from the following IP addresses or address range field by clicking the X sign next to it. Use the Save button to save your changes (Fig. 2.).

Fig. 2. Removing a blocked IP from the Connection filter policy settings.

Now, emails stamped with signatures should be delivered normally. If the issue persists, the IP address might be blocked by another connection filter policy or a smart host.

See also:

• How to check the location of a Microsoft 365 tenant and Microsoft 365 mailboxes
• How to prevent Outlook from adding signatures