550 5.7.1 Reject for policy reason

[Viking2]

Have been getting error messages when sending emails.

Undeliverable : 550 5.7.1  Reject for policy reason.

Have checked with recipients and emails have not been blocked their end. 

This is happening on-and-off for the last 5 days.

What is the fix for this please.

[Flaneuse]

I've had this too. There's a reply by the experts on a post by teegeepee65 from yesterday (11/02) giving a fix  - haven't tried to see if it works yet! Seem to be able to send to gmail addresses ok.