I have a user that has problems sending interoffice emails. Their mail is on an Exchange server. They are at a remote location and communicate with the server via POP and SMTP. When he sends an email to other staff in the same office, it never reaches them. I checked the Junk folder and did a search of all folders. I also whitelisted him.
If he sends an email to someone outside of the office and CCs his staff, they receive it. The Exchange logs shows that the emails are being sent.
I had him send an email to staff that was located in the city where the Exchange server sits. His mail ended up in Suspected Junk.
This one has me stumped.