What is SMTP?
SMTP is an acronym for 'Simple Mail Transport Protocol' - it is the standard mechanism used by most email programs and servers to send mail over the Internet.
SMTP normally runs on TCP port 25.
SMTP is a 'push' protocol so you cannot pull emails off the server using SMTP. However, by using ETRN (extended TURN) one server can request another server to send it's waiting mail to it - this is typically used when the receiving server is not permanently connected to the Internet.
The SMTP protocol started as purely ASCII (text) based and did not handle binary files properly. To resolve this MIME / UUENCODE is used to encode binary files so they can be sent correctly as email attachments.