Originally SMTP had a very limited set of commands (HELO, MAIL, RCPT, DATA, RSET, QUIT, NOOP) but as of RFC 1869, an extension mechanism was introduced. The initial HELO command was replaced by an EHLO command and the response to an EHLO command contains all the extensions the server supports. These extensions are identified by an IANA assigned name.

Extensions are used for example to implement inband authentication (AUTH), explicit message size limitation (SIZE) and explicit queue run initiation (ETRN). Each extension might add new command verbs, but might also add new arguments to various SMTP commands. The SMTP proxy has built in support for the most important SMTP extensions, further extensions can be added through customization.