Microsoft on-premises server product that runs Office Online.
Hello Mary Owen
Thank you for posting question to Microsoft Q&A Forum.
May I know if you are having a personal account (@outlook.com, @hotmail.com) or a business account (@domain.com) ?
If this is a business account, you could ask your administrator to sign out your account from all devices, change the password and enforce Multifactor authentication (MFA).
If this is a personal account, please change the password, also enforce MFA and sign out from all devices.
When you failed to send an email, you should receive a bounce-back email indicating that this email could not be delivered - could you share me a screenshot of the error message?
I also initiated a private message so you could share it with me - this should help me provide you more accurate troubleshooting steps.
I am looking forward to your reply.
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