Using SMTP to receive mail Simple Mail Transport Protocol (SMTP(RFC821)) is a server-to-server protocol for sending e-mail across the Internet. Briefly, a mail client will make a TCP connection to an ISP's SMTP server and upload a mail message (complete with headers) and instructions to whom the message should be delivered. The SMTP server will then either deliver the message (if it knows the final recipient) or pass it along to another SMTP server. SMTP works best when all servers are connected all the time. If the receiving server is not available, then the sender will have to queue the message and try later. Eventually, the sender will either make it through or give up and return the message to its originator. In the case of dial-up connections, the receiver may be unavailable more often than not. More: Advantages of using SMTP Disadvantages using of SMTP