User cannot send email internally or externally but can receive email.
I have attached a screenshot is the error the user is seeing and also added the warning from the mail flow troubleshooter.
5.7.1: The sender is not authorized to send e-mail messages to this e-mail address. Possible causes:
1)The session is not authenticated and does not have the 'ms-Exch-SMTP-Submit' permission.
2)The session is authenticated but does not have the 'ms-Exch-SMTP-Submit' permission.
3)The 'MAIL FROM' command specified an address at an authoritative domain and the session does not have the 'ms-Exch-SMTP-Accept-Authoritative-Domain-Sender' permission.
4)The authenticated user does not have permission to submit on behalf of the sender address specified in the header of the message and the session does not have the 'ms-Exch-SMTP-Accept-Any-Sender' permission.
5)The recipient domain does not match any of the accepted domains and the session does not have the 'ms-Exch-SMTP-Accept-Any-Recipient' permission.
6)The recipient domain is rejecting all mail from this server using a block list.