problem with mail to a email list not arriving

I hope someone can throw some light on this problem.

One of the email lists I manage using Mercury seems to have a problem. There are only 8 members in the list, but when I send a message the
following  shows up in the Mercury E logs:

451.4.3.0 multiple destination domains per transaction is unsupported

then the message appears at my end to have been sent out successfully, but is not received by the members.

This list has worked without a problem for many years, as have the other lists I support.

The message seems to appear after a group of gmail accounts have been prepared for transmission, so I tried separating the gmail addresses and
the others into two separate lists to be sent separately, but that didn't seem to make a difference.

I have looked up the description of the 451 item, but that didn't seem to bear any relevance to the issue.

Has anyone else seen this problem, or have a suggestion as to what the source of the problem is.

Any ideas welcome.