qmail or postfix

["Gary Smithe" <deb.guy@gmail.com>]

Hello all,

I work at a small ISP with a few dozen email hosted domains.

Our primary server is a redhat box running qmail and vpopmail.

I'd like to switch to a debian box, but have a question that hopefully
someone can help with.

>From what I've read, qmail and its tools break several Linux
conventions, so I'd like to go to postfix.

Here's the question:

What are your opinions of qmail / vpopmail (i.e. should I even switch
from them)?
If I do switch, what is the best replacement for vpopmail (where I can
use maildir's and copy the contents from the old mail server to the
new)?

Thanks In Advance

Gary