Mail server design considerations

Creating a decent mail server is more difficult than it first appears. There are a number of issues which require consideration:

• no mail that has been accepted should be lost if power is lost or the server is restarted
• mail messages should never need to be kept fully in memory - they can reach many MB each
  
• only one copy of each message should be stored on disc, regardless of number of recipients
• the delivery of messages for remote hosts that cannot be delivered now (e.g. due to network problems) should be reattempted after a delay of a set number of minutes or hours
• each message should get a unique id so it can be tracked by the administrator

Kamaelia mail server

I have made a simple mail server for Kamaelia. Currently it can receive mail and store it but not forward it on. See /Sketches/RJL/SMTP/ in SVN.
The server's design follows the normal SimpleServer format except that each SMTPServer instance links itself to a shared QualityStorage component which handles saving messages (the 'quality' being that it confirms write-completition). A separate delivery component copies (hard links) messages into local mailboxes.

Mail server topology (when finished)