Check whether the service or the device is affected
Open webmail in a browser and test the same account there. If webmail works but Outlook does not, the mailbox is probably available and the problem is more likely to involve the local application, profile, cached credentials or device. If webmail also fails, check the account and service status first.
Read the error before repeatedly signing in
A password prompt can be caused by an expired password, MFA requirement, blocked sign-in, stale token or damaged profile. Record the exact message and avoid approving unexpected MFA prompts. Repeated prompts you did not initiate may indicate that someone else has the password.
Confirm sending, receiving and synchronisation separately
Send a small test message to an external address and back again. Check the Outbox, Junk Email and mailbox storage. A message stuck in the Outbox, a full mailbox and a mail-flow problem require different fixes.
Do not delete the profile or data file too early
Recreating an Outlook profile can help, but confirm that mailbox data is safely stored online first. POP mail, local archives and unsynchronised folders may exist only on the computer. Back up important local data before removing profiles or data files.
When to escalate
Seek help if several users are affected, the account shows unfamiliar sign-ins, email forwarding rules have appeared, messages are being rejected by other organisations or domain and DNS changes may be involved. These can require Microsoft 365 administration, security containment or mail-authentication checks.
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