Outlook Desktop won't send from SharedMailboxes after enabling "SendFromAliasEnabled"

[RodoMCP]

After enabling the feature "-SendFromAliasEnabled" in Exchange Online Powershell, Outlook Desktop won't send emails from sharedmailboxes anymore. Error: "You do not have the permission to send the message on behalf of the specified user"

I'm assuming this is a name resolve error, because if we click on the "From" field and look for the account in the GAL, it does send the email without any issue. However, typing the address in the "From" field is when we get this error.

Any help will be appreciated.

[#2]

Seems to work fine here. You should always select the address from the GAL, do not use the autocomplete cache or any other stored entry. @Gabe Bratton just in case.

[#3 · Accepted solution]

@Vasil Michev Actually I found the issue.

But,

• I was not using Outlook Autocomplete
• Outlook cache was deleted.
• Profile remade

... and still had that problem.

Solution: I realized when typing the address in the "From" field Outlook was resolving vs. the Offline GAL, but these sharedmailboxes were not syched to the Offline GAL.

Once I fixed that, there no issues sending from Outlook Desktop.

If anyone have same issue after enabling send from alias in Exchange, make sure the sharedmailboxes are synched to the Offline GAL. At least this worked for me.