External sender has tried repeatedly to send email messages to multiple recipients at a domain hosted through Office 365. The messages never appear in the recipients' Inboxes or Junk Mail folders; the sender never receives an NDR or any indication the message was not received. Recipients have added the sender's domain to the "Safe Senders" list in OWA options, but it's as though they are getting filtered prior to reaching the mailbox.
We are using Office 365 P1, so we have no insight into what may be going on at the SPAM filter level. I have seen several other posts about this that simply end with a MSFT agent sending a PM to the complaintant, so I have been unable to resolve this on my own.
Thank you in advance for any suggestions!
