Start with the right checks.
The useful first question is whether one message or all outgoing mail is affected. A large attachment and an unavailable connection need different fixes.
What to do next.
Check the account and connection status in Outlook.
Compare a small harmless message without an attachment.
Review the failing message’s attachment size and the provider’s limit. Consider an approved sharing link if appropriate.
Keep the original content saved and ask the recipient through a known channel whether a consequential message arrived.
Check the result.
Confirm delivery of the harmless test and review the original Outbox entry. A Sent Items copy alone may not establish that the recipient received the message.
Stop and get help if…
Several messages fail, the provider rejects the address or credentials are repeatedly requested. Share the error text without including passwords or confidential attachments.