Strange MAPI/Exchange/IMAP behaviour in OL 2016 under10.0.19044.2006


Outlook version
Outlook 2016 32 bit
Email Account
Hello everybody!

I have asked this a while ago on but never got any reaction. So I guess it's alright to take another chance here.

We are seeing an odd MAPI behaviour recently, maybe somebody here can shed some light on it...
It seems that this has started showing around the time of our latest Windows or Office
updates but we cannot really confirm that as the person using their system in the described
way does not do so every day and might have ignored the problem for a while as a mere annoyance.

Software involved:
Windows 10 Professional 10.0.19043.1826 (updated to 10.0.19044.2006 by now, no change of symptoms)
Financial software Lexware Professional 2022 22.08
Outlook 2016, part of Microsoft Office 2016 Professional Plus 16.0.15330.20266 (32 Bit) with two Outlook
accounts configured: (Exchange) and (IMAP).

For organizational reasons Lexware sometimes needs to send mails as user1 and sometimes as user2.

Case 1: Exchange account ( is the default account, mail is sent flawlessly:
A new message window is opened in Outlook with the sender set to
1a: Clicking on "Send" sends the message and stores a copy in "Sent Items" of the Exchange account.
1b: Changing the sender to sends the message and stores a copy in "Sent" of the IMAP account.

Case 2: IMAP account ( is the default account:
A new message window is opened in Outlook with the sender set to
Clicking on "Send" results in error message "Sie besitzen nicht die Berechtigung, die Nachricht im
Auftrag des angegebenen Benutzer zu senden" ("You do not have the permission to send the message
on behalf of the specified user").
If you now use the drop-down (labeled "From" or "Sender" in the English GUI?) in the message window
to change the sender to and immediately back to,
and the click "Send" the message is sent (as user2) and a copy stored in "Sent" of the IMAP account.

When sending messages manually both accounts behave as expected i.e. messages are sent from the respective accounts and
copies stored in the expected folders. Both accounts are to the best of my knowledge not connected in any other way aside
from being configured in the same client.

So the question is: why do we need to switch the sender twice if the default account is set to the IMAP account?
(And why does that even work?) We would obviously like Outlook to send from the default account without requiring
any additional user action.

I first thought that was a problem with the third-party software (Lexware) and fiddled with a few options
there unsuccessfully.

Then I tried to use Word to send a message and noticed a puzzling behaviour there as well:
With the standard account set to IMAP ( Word opens a new message window in Outlook and sends
the message as user2. However I need to do the same sender switching as described in case 2 above to have
Outlook store a copy of the message in "Sent" of the IMAP account. Not switching the sender to user1 and
back to user2 does send the message but does not store a copy.

I enabled transport logging but seeing OPMLog.log being full of "Error code = 0x00000000" entries this
doesn't seem to be very helpful as well.

I admit that I use Outlook only when I'm told to do, so I might just be missing some point here but the
described behaviour seems very odd to me.

Any idea?
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