This is a very specific e-mail (de)multiplexer. It's main use case is to filter all e-mail for a domain through a single e-mail address.

This script is installed as the handler for all mail on a domain pre-domain and as the handler of all mail for a single post-address. An e-mail handled for foo@pre-domain is tagged and forwarded to filter-address. If this tagged e-mail is returned to post-address, it is forwarded to foo@target-domain.

(I assume a vpopmail/qmail setup)

Suppose you've got a domain mydomain.com and you want all e-mail sent to mydomain.com to be filtered through a single GMail account myfilter@gmail.com.

Create two new e-mail domains: post.mydomain.com and real.mydomain.com. Install real.mydomain.com as if it was mydomain.com. Set the .qmail-default for mydomain.com to:

| /path/to/pymm.py pre -f myfilter@gmail.com -t real.mydomain.com -p mydomain.com -P post@post.mydomain.com

Set the .qmail-default for post.mydomain.com to:

| /path/to/pymm.py post -f myfilter@gmail.com -t real.mydomain.com -p mydomain.com -P post@post.mydomain.com

And finally configure myfilter@gmail.com to forward all e-mail to post@post.mydomain.com and you're all set.