Email sent from Shared Mailbox gets stuck in Outbox

armandsalmon Hey, so we managed to get this resolved using the solution below. Since we applied this, it's been 2 months without issue. I hope this helps you as well.

Resolution

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 https://can01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.mysysadmintips.com%2Fwindows%2Fservers%2F373-connect-powershell-to-exchange-online-office-365&data=05%7C02%7CMike.Olejnik%40saultcollege.ca%7C3c662b4353344d1cd1ee08dcbbc281ec%7C9c61dd9d3b294f6db3b1f212de3c25da%7C0%7C0%7C638591691275807068%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C4000%7C%7C%7C&sdata=pd5Ecs6qk0h0J2zHahdvVayzK8783b0rqeMqOxGXYXc%3D&reserved=0.
• Run: Remove-MailboxPermission -Identity mailto:email address removed for privacy reasons -User mailto:email address removed for privacy reasons -AccessRights FullAccess
• Run: Add-MailboxPermission -Identity mailto:email address removed for privacy reasons  -User mailto:email address removed for privacy reasons  -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).