So we continue to have a disaster with Office 365 Shared mailboxes in general... Windows clients mainly being impossible to get to work in the simplest "does what it is supposed to" fashion. All we want is simply to share several email addresses, to have multiple user access to them, and to have Drafts/Sents end up in the boxes of the actual shared account
Initially we tried the 'default' - automapping shared folders/accounts & enabling various combinations of 'Send As' and/or 'Behalf' for users. Struggled with the above working at all - various permissions errors and sync errors... Deleting and recreating profiles from scratch per some suggestions was to no avail.
After reading many threads about this, ended up:
Deleting and recreating profiles from scratch again.
Adding each shared mailbox individually to the mail profile. So mail profile has "RealUser", "Shared1", "Shared2" as 'accounts'. Shared accounts use RealUser credentials.
Ensuring shared mailboxes are using "Cached Exchange Mode" (per several threads)
Running PowerShell permission commands manually (per a number of threads)
All folders sync perfectly to client both directions
Creating draft from the shared account works perfectly (saved to SharedUser/Drafts)
Sending the draft ends up with message that sits in Outbox for all eternity
Going into the shared mailbox via OWA (as same end-user account), can create or open existing drafts, and Send perfectly fine
Mobile apps work absolutely perfectly. Read/draft/send email from any of the shared boxes just as they should
What is wrong with the Windows Outlook client? Why is this so difficult for what should be the "core" desktop client application!?