We ARE on 0365. The issue is on the recipients. not our sender accounts even with the DMARC. It never reaches the recipient because even with a whitelisted domain, many firewalls are rejecting the ip source. For instance, our current public IP just changed to 166.137.105.197. You can see from this example, that IP is flagged on blacklists RATS and Spamhaus. Spamhaus exceptions update quickly but we email a lot of vendors all whom have different policies in place. It's tedious and as soon as they whitelist, the IP changes. The rules of mail flow in 0365 on recipient end (for those that are also utilizing it) only allow an IP range for exclusion; there is no defined range with ATT that can currently be provided.  https://mxtoolbox.com/SuperTool.aspx?action=blacklist%3a166.137.105.197&run=toolpage