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One of our Users is having a strange issue, before explaining the issue I should clarify that our user Exchange server is collecting mail using a POP3 connector. (not sure if it's relevant)

The user having the issue can receive all external. However 2 accounts are unable to send to her. If I send a message from one of these accounts and cc a working account then only the working account receives the message and only if it's fwd'd to the user does it come in. As I said above it's a POP3 setup, so I tried created a forwarder and tested that in the same way and the result was the same.

I've tried removing her local user profile, reintsalling Outlook, setting account up on another user account. All had the same result.

Any ideas where to go from here?

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Do you receive an NDR on those mails not reaching the user? If you receive an NDR check if Exchange complains about the recipient like this IMCEAEX-_O=DOMAIN_ou=first+20administrative+20group_cn=Recipients_cn=currentmailaddress.

If you don't receive an NDR you could still check the headers of the email sent in the first place for that in the To field. It is possible that if the Exchange has been migrated from an earlier version to the current installation that some old emails and sometimes also the global address book still contain an X.500-address for that user.

Outlook will try to use the X.500 address (even if you type in "user@domain.local" in the To field) to send mail to that user while he/she does not have that address any more. Thus exchange cannot find the recipient and generates an NDR to the sender. You can manually add an X.500 address to the recipient as described in this knowledge base article if this is the case for you.

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I would connect to your source mailbox (the POP3 one) using Outlook and totally bypass the Exchange connector for a bit. I'd then perform your test again to verify the emails are definitely coming in to the mailbox.

It's possible the message is being rejected by the source mailbox because it looks spammy, or may even be going into a junk folder in the POP3 mailbox. I'll admit I don't know the intricacies of Exchange POP3 connectors, but if it's only fetching messages from the inbox folder of the POP3 account, and some mail is going to a junk folder, that may account for the user not seeing the mail in Exchange.

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