553 Relaying Disallowed

553 Relaying Disallowed

One of our employees is getting this message quite frequently over the last week.
He's using Outlook to send the emails and has not had any problems until recently.
Can you help me?

Your message did not reach some or all of the intended recipients.

Subject:    Subject
Sent:    13/07/2017 15:05

The following recipient(s) cannot be reached:

' abc@def.com' on 13/07/2017 15:05
553 Relaying disallowed as "John Smith" < zyx@wvu.com>