It’s likely that you have enabled your secure e-mail address in the Office settings. When you send e-mails from this address, you may receive error notifications such as this:

The mail system 
<>: mail transport unavailable 
Content-Description: Delivery report 
Content-Type: message/delivery-status 
X-Postfix-Sender: rfc822; 
Final-Recipient: rfc822; 
Original-Recipient: rfc822; 
Action: failed 
Status: 4.3.0 
Diagnostic-Code: X-Postfix; mail transport unavailable

When you are using your secure address for e-mailing, we have to employ a secure connection to transport your e-mail to its destination. Unfortunately, not all recipient servers support secure connections. We repeatedly try to send an e-mail for three hours, after which we cancel the delivery attempt and send you this notification. If you absolutely need to use your secure address, let the recipient know about the delivery issue so they can get their mail server configured accordingly.