We are aware of an issue affecting the delivery of some emails sent to Office 365 addresses. Specifically, emails sent from one of our IP addresses (3.19.87.175) have been temporarily blocked by Microsoft's email security system.

Office365 show the IPs isn't in their blocklist

The best / most foolproof way to avoid wrongful "banned sending IPs" is to create a Connection Filter allowing the SpamTitan IPs

1) Create a Connection Filter in Office365 for our IP addresses.  This bypasses our IP address from Office365 spam checks (which is advised anyway):

https://learn.microsoft.com/en-us/defender-office-365/connection-filter-policies-configure

These are our IP addresses to include (same ones as the cloud11/cloud12 MX records):

3.19.87.175

18.217.94.247

18.223.46.43

==> this is Connection filtering and is part of Office365 AntiSpam Protection, not the Partner Organization to Office365 Connector.

2) As per the delist request, it also does no harm to create a support ticket from your recipient tenant (since the delist doesn't think the IP is banned, only some servers).