Hi,
Start your troubleshooting by following the guides below:
My email message doesn't send
https://support.microsoft.com/en-us/office/my-email-message-doesn-t-send-09d51757-89e8-4b91-b732-a313a36f1e0d
My messages won't send
https://support.microsoft.com/en-us/office/my-messages-won-t-send-97748418-bbd5-4743-a05b-581f22a466dd
I can't send or receive messages with my email account
https://support.microsoft.com/en-us/office/i-can-t-send-or-receive-messages-with-my-email-account-2d123bee-676c-4279-bce2-9b4c2da1e8f5
Best regards,
Leon