Hi Felicity,
Which type of email account do you use? Exchange, IMAP or POP3? Is it an Outlook.com account you are using?
Could you provide the entire text of the bounce back message so that we can look into it and see whether there is any other clue?
Meanwhile, please also try to exit Outlook, logon to the webmail interface of your email account and see whether the emails go out successfully. This will help determine whether the issue is related to the mail server.
Regards,
Steve Fan
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