Distinction Between Message Submission and Transfer Will Become More Important

Message submission is when an email client submits a new message to a server. Message transfer is when a message is moved between two email servers. Internet messaging uses one protocol (SMTP – Simple Message Transfer Protocol) for both functions. 

The distinction between message submission and transfer was formally added to the Internet messaging architecture in the 1990s, and is formalized in RFC 2476 ("Message Submission" 1998) that defines rules for using SMTP to submit messages, including a preferred port (587) different from the standard SMTP port used for transfer.

This excellent specification is not generally used. Most email clients default to using port 25 for submission. Most email servers are set up to accept SMTP on port 25 from clients and other servers. This will change (but not quickly).  Making a more rigid distinction between submission and transfer, and applying clear rules for both, is going to help in the fight against spam and other email abuse.   

Steve Kille

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