Check permissions by IP address in Microsoft 365 :
In order to bypass SPF checks, we strongly recommend that you enter our IP range on the tenant Office. This improved filtering will allow to bypass the e-securemail IP as the legitimate sending server of the sender and avoid the errors visible in the headers (SPF error, because it takes as sending server, for example, the domain example.com, an IP of e-securemail that does not match their SPF). This results in a wrong categorisation of the email in the user's inbox.
- Go to the M365 security portal - https://security.microsoft.com/
- Open Email & Collaboration in the left sidebar and go to Policies & Rules > Threat Strategies > Anti-Spam Software
- Click Connection Filter Policy, then click Change Connection Filter Policy in the drop-down menu
4. Add the following IP addresses to the Always allow messages from the following IP addresses or address range field
For the IP Ranges to be filled in,
please contact our support department at support@secuserve.com.
Practical link: https://docs.microsoft.com/en-en/exchange/mail-flow-best-practices/use-connectors-to-configure-mail-flow/enhanced-filtering-for-connectors