Separate three different failures
A message stuck in Outbox may never have reached the server. A message in Sent may still be rejected later. A bounce is a delivery report and should be read for its specific reason, not treated as the same problem as a connection error.
Try one controlled test
- Send a plain-text message without an attachment to an address you control.
- Check the sender address and remove mistyped recipients.
- Compare the result with official webmail.
- For an app-only failure, verify the outgoing hostname, encryption and authentication against provider instructions.
Follow the error category
Authentication errors require supported sign-in. Message-size errors require a smaller attachment or another sharing method. Recipient failures may require correcting the address. Provider sending restrictions must be handled with the provider.
Do not repeatedly resend a large queue; this can create duplicates. Preserve a copy of the draft before making changes.
Explore provider-specific guides for official configuration references.