Emails being blocked to certain addresses by uri.cal1.sophosxl.com

For some time now, some emails that I have been sending from Outlook are bouncing.

At the moment an email arrives informing me of the following error:

554 5.7.1 Service unavailable; Sender address [xxx.yyy@xyz.com] blocked using uri.cal1.sophosxl.com
Does anyone know how it can be solved?