Description of Problem:

If a POP3 server has 'Leave Messages on Server'
enabled, the current implementation will not
remove 'Expunged/Deleted' messages from the POP3
server.

However, modern POP3 servers that implement the
UIDL verb, with a bit of intelligence on the
client side (aka Evolution) can be made to purge
Expunged messages.

Steps to reproduce the problem:
1. Enable 'Leave Messages on Server' for a POP account
2. Delete one or more messages and expunge.
3. Review contents of POP account via a secondary
POP client (e.g. Netscape Mail).

Actual Results:

Note that all messages are present, included
supposed Deleted/Expunged messages.

Expected Results:

Expunged/Deleted messages will be removed from
POP3 server at periodic intervals (e.g. next
connection), prevent spool over-consumption.

Additional Information:

http://www.faqs.org/rfcs/rfc1939.html

Possible implementation:

- For each message RETR'ved from the server,
  fetch and store the UIDL as a part of the 
  set of metadata stored for each message.
    Note: UIDLs in local store should be indexed
    to minimize performance issues.
- To determine messages to DELEte, compare 
  the set of UIDLs stored on server with the
  total set of UIDLs stored locally - each 
  UIDL on the POP3 server which does not exist 
  in the local set of UIDLs is a DELEte 
  candidate.
- Iterate over DELEte candidates, removing 
  each from the POP3 server.
- QUIT session, updating remote store

Possible design issues:

- current local store metadatabase cannot 
  be easily extended to store UIDL attributes 
  for each message held in local store

Possible run-time issues:

- POP3 server does not support UIDL verb

User control:

- Per POP server, enable/disable removal of
deleted and expunged messages.