I figured it out. A new domain had been added and the Exclaimer connector hadn't updated so was dropping emails. Refreshed Exclaimer and all was ok.
Thanks for looking.
Hi all,
I have a Plesk server using Postfix, and email is disabled on all domains. Outbound email is allowed and is working to a point.
I have a group of sites that all share the same 365 tenant as they are in-house domains. I have set Plesk DNS name and rDNS up and mail to gmail etc flows...