Email Replies Undeliverable

I was having issues on 2 computers with em client running super slow.  After creating Exchange accounts my computer does well but my wife’s computer is having issues.  We both use Outlook.com for our mail.  When she sends an email the recipients receive it fine.  When they try to reply the get this message, “Delivery Status Undeliverable - Address not found outlook 7C1AEF2F74FD02C7@outlook.com because the address couldn’t be found. Check for typos or unnecessary spaces and try again.”  If they send her an email in a new email it arrives fine.  My computer does not do this.  Any suggestions would be appreciated.

Hi,

Have you tested if this issue occurs even with your account when you reply to mail from the same sender?
Are you able to send emails from that account via webmail? If not, please try signing to web interface an try it there.
If the issue persists it may be an issue with ESP or there is mistype in email address.

Kind Regards