[Exim] more thoughts on moving to new server with maildir

I just had a thought, can I get comments?

I have the old mail server, mail1 let's call it, and mail2 for the new
one.

how well would keeping the MX record for the domain set to mail1, and then
use some sort of spam tool to process the mail on mail1, and the users are
only allowed to mail2 (webmail server) to read mail, and when the login
into mail2 it pulls the mail from mail1 moving it into maildir format. Is
this a good idea, bad idea, half a good idea?