Hi Aaron,
I'd like to confirm the following things:
1. Does this issue occur when sending emails from Office 365 to AOL, or the reverse situation?
2. Because Yahoo! also rejected the email, please confirm that whether the error message or error code is the same.
3. According to the error message you provided, there is a problem with the separate recipient's mailbox, I want to know whether you can send emails to other recipients/email addresses.
In this situation, if you choose the email recipient in Outlook desktop client using AutoComplete list, I suggest you clear the AutoComplete list cache in Outlook client and then check to see if the issue persists.
The detailed steps: How to reset the nickname and the automatic completion caches in Outlook
Thanks,
Bourne Zhang