Hi,
These articles may help you.
My messages won't send:
https://support.office.com/en-us/article/My-messages-won-t-send-97748418-bbd5-4743-a05b-581f22a466dd?ui=en-US&rs=en-US&ad=US
Outlook.com won't let me send an email:
https://support.office.com/en-us/article/Outlook-com-won-t-let-me-send-an-email-62de1f9b-6552-4239-8578-078d292bb31a
In addition, as this issue is mainly related to Outlook, I suggest discussing it in our Office forum. They are the best resource to troubleshoot this issue.
https://social.technet.microsoft.com/Forums/office/en-us/home?category=officeitpro
Best Regards,
Tao
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