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