pyWM can operate in two modes: trusted and public.
By default, every installation is "public."
Public: Locally-cached messages are deleted upon
logout or timeout.
When is this a good idea?
The mail server is on a very fast connection, or is localhost. You
don't want two copies of the message on disk if it's very easy to
retrieve.
You're providing services to an offsite mail server, but you're
potentially going to have lots of users. You want to free up
disk space as soon as possible, users be damned. You'll have to
download that 4MB message for them again next time they log in, but
you can't afford the keep it on disk.
Trusted: Locally-cached messages are kept on
disk until they're deleted from the POP3 server.
When is this a good idea?
You're providing webmail services for yourself only,
or just a few users, and the mail server is offsite on a slow
connection. You don't want to have to download that 4MB attachment
again next time you log in.