Outlook 2016 + Exchange 2016 (latest cu)
One user. Opens a new message in Outlook.
Sometime the to field is changed and that is the reason why the mail got stuck in the outbox. It is never sent.
Example:
I create a new message.
I fill in the to-field with info@domain.com. I have some words in subject-field and some content.
I click on send.
Error comes that it is not sent.
When I open that "sent" mail I see in the to-field:
'=SMTP:info@domain.com'
It has to come from somewhere. Yes that e-mail address is in the cached mail but when I open the stream_autocomplete I see no fault. See screenshot in attach.

Searches on the internet (I am not alone) and possible results does not result into a solution.
Also this: we have a citrix environment. Users are logging on but doesn't know on which server they are. I know the cached file is in the roamcache folder and is a local item. But clearing it and starting over keeps giving errors (sometimes).
What I see indeed is that there are more than one stream_autocomplete file for that user but that is probably because she has more than one mailbox in their Outlook profile.
I hope to see some hints that maybe can lead to a solution. It would make my day.
kind regards
Kurt