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.
