If MercuryE logs show that messages have been accepted for delivery by the receiving server this is a problem at the receiving end, most likely due to spam filtering or similar. Try contacting the helpdesk at the receiving end and ask them why messages aren't delivered.

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.

There is some info on this here: https://kc.mcafee.com/corporate/index?page=content&id=KB83946&snspd-0315

Have you tried the option to explode the submissions on the mail list's Distribution tab??

I'm no expert, but the issue seems to be multiple addresses being handled by a single SMTP session which are not allowed to be relayed by the receiving server(??) 

I am very happy to be corrected!

Mercury will by default attempt to group all recipients with the same receiving SMTP server together in one transaction instead of starting multiple transactions. Google handles email for extremely many domains besides gmail.com. The MercuryE log file should show exactly what happens.

4xx series errors aren't permanent, which means that Mercury should make additional attempts to distribute the message to those recipients that weren't accepted in earlier connections. This should as well show in the log file.

That said, there is also a huge risk that mailing list messages are tagged as spam by Google's server, so list members may want to check their spam folders.

Many thanks for the various pointers to help solve the problem.

I have followed up the various suggestions, but so far to no avail, Direct email to/from individual list members works ok. Checking with ping and traceroute also shows no problem, I have also changed the DNS servers in the mercury configuration from the Windows default to OpenDNS instead (shows no change). Outgoing email to the list via MercE appears to work ok despite the 451 message. So in summary, the problem is that group email sent to a one particular list does not reach the recipients, despite apparently being sent correctly by Mercury. Anyway I can manage for the moment using pmail with the list members entered as BCC, although of course that does not directly allow for the normal communication within the list that should be provided by Mercury.

 The only other possible source of the problem I can think of might be if something has changed in my ISP's configuration, but I haven't been able to identify anything there yet.