Outlook - I can reply as a shared mailbox, but I can't send a new email from the shared mailbox

James Moore 5 Reputation points

I've checked that I have SendAs permissions, and if I send an email to the mailbox, I can reply just fine, but when I compose a new email and send I get " This message could not be sent. You do not have the permission to send the message on behalf of the specified user. "

I have given myself full control and send as permissions and do not have send on behalf permissions.
I can also send email from the shared mailbox just fine using the web interface, but not the outlook program on my PC. I have attempted to search online for answers and this only happens in the outlook application. Another user is experiencing the same issue when trying to send a new email from a shared mailbox that was recently created. my version of outlook is Microsoft® Outlook® for Microsoft 365 MSO (Version 2309 Build 16.0.16827.20130) 64-bit. Any help would be appreciated, I have attempted to rebuild the outlook profile, redownload the offline address book and redownload the current offline address book.

A suite of Microsoft productivity software that supports common business tasks, including word processing, email, presentations, and data management and analysis.
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