Are you sending to the same email address every time it bounces back?
I am still here.
Are you selecting emails from a list, typing them yourself, or replying to an email when it happens?
Generally, the issue happens when selecting emails from a list/group and one of the email address has something wrong with it.
What happens if you try to send an email to yourself?
If you send an email from the WebMail portion (not through Outlook), will it send?
Great, if you open the WebMail (through IE, Firefox, or Chrome) and send an email, does it work?
Let me know
Okay so we have narrowed down the issue to Outlook 365 and Earthlink.
What did Earthlink say about the issue?
Try disabling your security system/email scanning feature.
I apologize -- Are you there?