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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: <user@server.com>
250 2.1.0 <user@server.com>... Sender ok
RCTP TO: <anotheruser@server.com>
250 2.1.0 <anotheruser@server.com>... Sender ok
DATA
354 Enter mail, end with "." on a line by itself 
From: <user@server.com>
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.

user590028
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2 Answers2

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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.

Ivan_Wang
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  • No plugins or email processing apps of any kind. Just office-365 and that's it. – user590028 Oct 08 '20 at 17:21
  • Try to do a message trace via Message Tracking Log(https://docs.microsoft.com/en-us/exchange/mail-flow/transport-logs/search-message-tracking-logs?view=exchserver-2019), maybe some **events(https://docs.microsoft.com/en-us/exchange/mail-flow/transport-logs/message-tracking?view=exchserver-2019#event-types-in-the-message-tracking-log) with sources(https://docs.microsoft.com/en-us/exchange/mail-flow/transport-logs/message-tracking?view=exchserver-2019#source-values-in-the-message-tracking-log)** caused this To field issue. – Ivan_Wang Oct 15 '20 at 09:28
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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.

user590028
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