stealing borrowing this information from a post on Sysadmin Tips:
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. Subject: Test Sent: 05/06/2018 19:47 The following recipient(s) cannot be reached: [email protected] on 05/06/2018 19:47 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.
The issue was resolved (with Microsoft support help) by re-applying Full Access permissions and removing shared mailbox Outlook auto-mapping for the affected users and then adding the shared mailbox manually in Outlook. Re-applying permission via Office 365 admin centre didn’t have any effect.
Launch Windows Power Shell as administrator and connect to your Office 365 tenant.
Remove-MailboxPermission -Identity shared_m[email protected] -User [email protected] -AccessRights FullAccessRun:
Add-MailboxPermission -Identity [email protected] -User [email protected] -AccessRights FullAccess -AutoMapping:$false
-AutoMapping:$false disables shared mailbox auto-mapping to users’ Outlook profiles.
Wait 30-60 minutes for changes to propagate until shared mailbox disappear from users’ Outlook.
Manually add shared mailbox to users’ Outlook: File > Account Settings > Change > More Settings > Advanced > Open these additional mailboxes.
Try sending email as the shared mailbox by selecting From: address from Online Global Address List (to prevent caching issues).