I am asking this question just to make sure that all bases are covered.
I have a client who has been in regular communication with a customer via email. One or two emails (out of dozens) haven't arrived. We look in the message tracking logs, do a variety of searches, and these emails do not appear in any log. Nothing in Quarantine, Junk Folder, anything.
If a message was received by the O365 servers and was blocked for any reason, shouldn't that show up in the log detailing why?
Is there some kind of 'perimeter' filter that may have blocked a message prior to it getting to a place where it shows up in my clients O365 logs?
The customer received no NDR, and when they resent (i.e. forwarded from their Sent Items) it did appear at my client.
The customer is at home and uses the email address supplied by her (big) ISP. Troubleshooting on that end is limited.
There isn't an issue with attachment size (one email had attachments, the other did not). The only thing that was a bit off was that it was a long email chain of a lot of replies back and forth. And, if it matters, there was some (very slight) language in the email.
Am I missing something here I don't know about O365?