SMTP Relay to 365

You can use FQDN in the pool member, from 11.6.0:

https://support.f5.com/kb/en-us/products/big-ip_ltm/releasenotes/product/relnote-ltm-11-6-0.html

"Populate pools by FQDN

This release includes the ability to configure a BIG-IP system with nodes and pool members that are identified with fully-qualified domain names (FQDNs). When configuring pool members with FQDN, addresses dynamically follow DNS changes. Fully dynamic DNS-managed pools may even be created."

I have never used, but should work fine. Don't forget to configure DNS servers in the F5s.