Mail Protocols

Mailkeep is capable of delivering mail to your servers using a number of standard protocols.

SMTP (Simple Mail Transport Protocol)

Mailkeep receives all mail using SMTP, which is the standard protocol for delivering e-mail on the Internet. Ideally, you want to host your own e-mail server which receives mail from Mailkeep using SMTP. SMTP is associated with TCP/IP port 25. MX mail servers must always listen on port 25 and for that reason if ISPs wish to prevent you from running your own mail server, they will block port 25. Mailkeep can, however, deliver mail to you using SMTP on a non-standard port.

ETRN (Extended TuRN)

If you connect to the Internet intermittently and wish to receive mail from Mailkeep using SMTP, you will need some way to indicate that you are ready to receive. The standard method for doing this is ETRN, which is a command sent using SMTP. If your e-mail server supports ETRN, you can ask it to trigger delivery by connecting to and issuing the ETRN signal (see Triggering Mail Delivery in the help section for more details).

ODMR (On Demand Mail Relay)

Your ISP may go a few steps further than blocking port 25 but blocking all incoming ports. Or, you may have a router or network policy that prohibits incoming mail connections. In either case, SMTP delivery to your mail server becomes impossible. This is where ODMR is suitable. ODMR is a method for trigger mail delivery, much like ETRN, except the delivery takes place within an SMTP transaction initiated by your mail server (rather than ours). This means that you can receive SMTP mail even if you cannot receive incoming connections.

If your ISP blocks outgoing connections to port 25, you can connect either to port 366 (the official ODMR port) or 2525 on

POP3 (Post Office Protocol)

If you do not want to use SMTP for mail receipt, or if your mail server does not support ODMR, you can collect mail the same way Mail User Agents (clients such as Outlook or Thunderbird) receive mail. There are many disadvantages to this method (you dont't have control over the mail delivery process or access to the message envelope) but ISPs are almost certainly not going to interfere with this. Also, it means that each of your users can collect their own e-mail (as we allow multiple POP3 collection from your account).

APOP (Authenticated POP)

APOP is simply a method for logging on to your POP account without transmitting the password. This is a secure handshake process that uses a hash method.