Office 365 - cannot send as shared mailboxes from Outlook
Some users started having issues sending emails from newly created Office 365 shared mailboxes. Users were added as shared mailbox members using Office 365 Admin centre. The shared mailbox would be automatically mapped in their MS Outlook 2016 as expected, but when users tried to send an email from the shared mailbox email address they would immediately receive below NDR:
Your message did not reach some or all of the intended recipients.
Sent: 05/06/2018 19:47
The following recipient(s) cannot be reached:
This message could not be sent. You do not have the permission to send the message on behalf of the specified user.
According to Exchange Admin Centre users had Full Access and Send As permissions to shared mailboxes. As a workaround, users were able to send emails as shared mailboxes using Office 365 WebMail.