external mail server and sending e-mails from a site problem

[dimas]

I have two servers with ISPConfig 2.2.25, perfect setup, Debian Etch.

The problem is the following.

One of my clients needed a domain, let's say, domain.com, and I bought it for him the usual way and set up nameservers ns1.myserver.com and ns2.myserver.com (these nameservers are my nameservers set up at my ISPConfig servers).

Everything was OK, the client's site was up and running, etc.

Then the client decided that he wanted to use another (external) mail server. At the time there were two A records in the DNS Manager:
xx.xx.xx.xx www
xx.xx.xx.xx mail
(where xx.xx.xx.xx is the IP address of my server and the client's site on it)

and one MX record
mail.domain.com

Now, I had to change the 'mail' A record to the IP address of the external mail server of my client, so now the two A records are:

xx.xx.xx.xx www
yy.yy.yy.yy mail
(where yy.yy.yy.yy is the IP address of the external mail server)

The MX record is the same.

Now, the client says that the external mail server works, BUT there is a problem - there is an html form on the client's site (hosted on my server) that uses php mail function to send mails to, let's say, user11@domain.com.

And these e-mails bounce, according to mail.log, - because user11 is not found! Apparently postfix looks for this user locally, and not at the external mail server. I wonder why this is happening and how can I change this behavior.

Please kindly help.