Forwarding mail to another MTA in postfix

I recently transferred to a new server (fasthosts 512Mb non-raid just wasn't good value for money compared to competitors).  The main pain in the neck is taking all the IMAP mail and sync'ing with the new server.

What i needed was a system to configure the new server to accept mail for the domains and have the old server fwd'ing incoming SMTP to the new server.  So when I change the MX recs no users lose mail.

Postfix's Transport directive takes care of this

vi /etc/postfix/transport 
your-email@your-domain.com        smtp:[10.65.43.210]

By putting the new MTA's IP address in square brackets we avoid a mx lookup and potentially the mail going round in circles.

All you need to do now is configure your new mail server to accept incoming mail from your old, i.e. accept it as a mail relay.  I moved from postfix to exim if my old MTA has IP address of 192.168.1.32 then I would add following to exim.conf as follows:

hostlist   relay_from_hosts = 127.0.0.1 : 192.168.1.32 

With this in place your old MTA will start forwarding incoming email to the new MTA.  All you need to do then is archive the users IMAP folders and copy to the  new server.