Plesk email after migration split on both servers

[coolemail]

The situation:

Server A is old server with FC2, Plesk 8.1.0. ns1.mydomain --> IP A

Server B has CentOS 5 with Plesk 8.6 and ASL. ns0.mydomain.com --> IP B

All domains have both ns0.mydomain.com and ns1.mydomain.com. I also have secondary name servers, and I am progerssively:
Telling secondary name servers to use IP B as the primary server.
Turning off DNS on Server A, so that it now shows only the name servers

Now, in the various email addresses, there is mail on both servers, and I want customers to get all the mail, no matter when the whois records are all updated, until everything is migrated and I will then hopefully be able to delete all clients from the old server.

Depending on the email address from which each email is sent, some is delivered to the old server, and some to the new server - any email sent from a domain hosted by this server will never leave the server.

I do not want to disable mail on the old server, or else they will be unable to get access to the emails. Nor do I want to disable the whole domain on the old server for the same reason.

Currently, I am advising them to go to webmail on each server and look at it that way, or change the incoming/outgoing mail server IP address to the IP of the old server.

Is there a way in which I can force ALL outgoing emails to go to the new server having carried out the steps above?

I hope I have explained that OK, and that someone will be able to help.

[Highland]

That's a very strange setup for DNS. It sounds like if I hit ns0 I get one set of DNS and if I hit ns1 I get a different set. That's a poor way to handle this. Worse, it puts the onus on your customers to work around this setup.

What you should do is make the MX records in both sets match. In other words, change the MX record on Server A to point to Server B. Or, better yet, have your servers mirror each other.