Why are my emails being blocked to certain addresses by uri.cal1.sophosxl.com?

For some weeks now, some – but not all – emails I have been sending from Apple Mail are getting bounced.

In my bin, I can see a Google red traffic light icon.

And also the following:

554 5.7.1 Service unavailable; Sender address [xxxxx] blocked using uri.cal1.sophosxl.com

How can I stop this happening?

Thanks