Feasability of E-Mail Laundering Service

I run a reasonable little mail server: postfix on point, followed in
close formation by amavisd, SpamAssassin, postgrey and pop-before-smtp
(all latest, I upgraded the lot last week). I'll probably install
clamav tonight and what breaks with *that*, lucky me.

A friend of mine has an account with an ISP whose spam filtering isn't
as good as mine. What I want to do (with her complete permission) is to
run a mail laundering service: I pick up mail from her ISP, process it
through my postfix et al, and deposit it into an account on my machine.
If necessary, I can have her username and password with the remote
service.

How feasable is this? What will I have to throw into the mix to make it
go?

-- Lorrie