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.