Protocol Interactions

There are four parts to an SMTP session:

1. Exchange relay identities.
2. Send the message envelope.
3. Send the message content or reset the envelope.
4. Release the session.

The actual commands used in SMTP aren't particularly interesting. What is interesting is how SMTP indicates whether a command succeeded or not (using three-digit reply codes) and two other commands in vintage SMTP, termed probe commands. The first allows you to verify the validity of a local mailbox at the server. The second allows you to expand the contents of a local alias at the server. In practice, neither of these commands is particularly useful. For "security" reasons most sites have disabled them.

In theory, this is an acceptable policy. Unfortunately, in practice the verify command sometimes gives out false positives or false negatives, depending on the implementation. As a consequence, an improved method of verifying a mailbox is to send an envelope and see if the recipient address generates an error. Either way, the envelope is then reset to avoid sending a message. Of course, not even this is reliable, since some servers are configured to accept all addresses and then later make a determination as to whether they are actually valid; if not, an error report is returned. SMTP has an extension mechanism allowing a standardized way for evolving the protocol. Here is a list of some of the extensions that have been standardized:

· You can indicate whether you want to receive delivery reports when a message is received at the final SMTP server.
· You can indicate how big the message is you're going to send. If the message is too big for the server, either due to a physical limit (not enough disk space), or an administrative limit (user over quota), then you'll be told not to bother sending the message.
· If you'd rather see enhanced error codes, rather than the three-digit reply codes, you can request that.

There are also some rather esoteric ones dealing with increased efficiency (command pipelining) or perceived efficiency (8-bit transfers), but these are of interest only to messaging software implementers.