Dynamics Community Forum Thread Details

Shared mailbox email works for one user but fails for others in Business Central

Hi everyone,

We are experiencing an issue in Dynamics 365 Business Central (SaaS) when sending emails using a shared mailbox configured as an Email Account. The same shared mailbox works correctly when emails are sent by one user, but fails when the exact same action is performed by other users.

Specifically, sending emails from user1 works without any issues. However, when sending from user2, the process fails with the error “Cannot submit message. Status code: 403”. When sending from user3, the error returned is “The mailbox is either inactive, soft-deleted, or is hosted on-premise. Status code: 404”. Both errors occur when using the same shared mailbox and the same Business Central email setup.

We have reviewed the email account configuration in Business Central and it appears to be correct. We do not see any differences in the Business Central setup that would explain why the behavior changes depending on the user, and the shared mailbox itself is active and available in Microsoft 365.

Additionally, since 2026-05-04, several email accounts that were previously working in Business Central have stopped working unexpectedly, which makes us wonder if there has been a recent change related to permissions, authentication, or the email integration used by Business Central.

Could anyone clarify why a shared mailbox might work for one Business Central user but return 403 or 404 errors for others, and whether there are any known changes or considerations that could explain why multiple email accounts stopped working suddenly from that date?

Thank you in advance for any guidance.

Categories:

Dynamics 365 Business Central

Hello,

This is most likely not a Business Central setup problem, because the same shared mailbox works for user1; with the Microsoft 365 Connector, Exchange still validates the actual BC user who sends the email, so user2/user3 must also have a valid paid Exchange Online mailbox/license and proper delegation on the shared mailbox, especially Send As permission, not only Full Access. The 403 usually points to Exchange/Graph refusing the send action because of missing permission, invalid mailbox/license, quota/policy, or access restriction; the 404 usually means the sender mailbox Exchange is trying to use is not a valid Exchange Online mailbox, inactive/soft-deleted, or hosted on-prem/hybrid. I would compare user1 vs user2/user3 in Microsoft 365/Exchange admin center: Exchange license, mailbox status, guest/delegated admin status, Send As delegation on the shared mailbox, and any Conditional Access or Exchange policy changes around 2026-05-04. If many accounts stopped on the same date, also check Microsoft 365 Service Health and tenant-side Exchange/Graph/authentication changes, because Microsoft Learn does not indicate a Business Central-specific setup change for that exact date.

Regards,
Oussama Sabbouh

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