Strange Outlook Addressing problems

Question

We are seeing a new problem with Outlook this past month. Outlook is generating email messages with valid envelope recipients, but INVALID Body Header recipients. Because a receiving user replies on Body-To to formulate a reply, any CC'ed users are unable to formulate valid reply-recipient list.

In each case, the users seeing this problem are hybrid Office-365 and Corporate-IMAP users. Here's a sample of the SMTP session we've trapped to demonstrate the problem

220 smtp.server.com ESMTP Sendmail 8.15.2/8.15.2/Debian-8;
HELO: LTUser
250 smtp.server.com Hello LTUser, pleased to meet you 
mail from: <[email protected]>
250 2.1.0 <[email protected]>... Sender ok
RCTP TO: <[email protected]>
250 2.1.0 <[email protected]>... Sender ok
DATA
354 Enter mail, end with "." on a line by itself 
From: <[email protected]>
To: "Another User"

Notice the "To:" header is their Display-Name and not the recipient's actual email address.

We've tried flushing PST/OST files, re-adding Outlook Profiles, flushing auto-completes entries.

It almost looks like Outlook is using a Display-Name from entries in the O365-GAL, but never bothering to formulate a valid recipient.

Answer 1

Did you encounter the same issue if you sent emails via OWA? If the issue only happened on the client, maybe some add-ins or software was affecting the mail flow, pls try to disable them temporarily and see if there is any difference.

Answer 2

After a month of support exchanges with Microsoft, we finally have a solution. I'm not suggesting the solution makes sense -- just sharing what Microsoft finally recommended for anyone else who stumbles across this issue.

First we re-went through the Auto-Completion entries and deleted all those for the afflicted recipient address. After they were all deleted, we completely typed the correct address, and then hit CTRL-K. That solved it.

If you google CTRL-K for Outlook it mentions something inane about causing Outlook to Check names (whatever that means). https://support.microsoft.com/en-us/office/keyboard-shortcuts-for-outlook-3cdeb221-7ae5-4c1d-8c1d-9e63216c1efd

In any event, this worked for our issue.