I need to have the emails that go from and to the tracker 
to be encrypted, so that nobody on the way can read it. 
Https is not an option. 
There could be a file with per-user passwords for email 
encryption. It should not be too difficult.

Best regards
  Dirk

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user_id=805804

Of course, common public key algorithms should be used for this.

How about this:
Whoever uploads his/her public key will receive encrypted
mails from roundup and may decide that the tracker should
accept validly signed mails (for this account) only.

The tracker instance would need a public key, too, to be
able to receive encrypted messages.

A later version might be able to get a missing key from a
public keyserver when receiving a signed mail.

There appears to be a Python crypto project
(http://www.amk.ca/python/code/crypto) which might be useful ;-)

Support for receiving signed or encrypted messages (with the assistance of pyme) has been checked in to CVS. Sending encrypted or signed messages is still not supported.

I've recently implemented encrypted PGP messages to and from the
tracker. This will be in 1.4.20.