SMTP Explained

SMTP – Simple Mail Transfer Protocol

Of all the web technologies out there SMTP is probably the one that has had the least changes, it is still pretty much as it was originally drafted. Sure the authentication changes from time to time but the core of SMTP has remained constant. It has had to adapt though, spam, blacklists and open relays have all had to be dealt with and it is this “bolt on” approach which has caused some unreliability to creep in.

What Does SMTP Do?

SMTP is the process by which mail is passed between servers, if you’ve ever sent an e-mail outside of your closed work environment then you have used SMTP. When you configure your e-mail client such as Outlook or Thunderbird you will be required to add an incoming server (this is usually POP3 or IMAP) and an out-going server, this will be SMTP. When you then send your e-mail it is delivered via that SMTP server to either the recipients server directly or via a number of inbetween servers. Generally speaking SMTP is not secure.

There are a number of standard ports over which a SMTP server listens, port 25 is used in most cases but ports 587, 465 and 2525 are also common. Problems can occur when an ISP blocks port 25 restricting users to using only their SMTP server. They do this in order to stop SPAM originating from their network but this kill all approach can cause issues for business users. The worst combination is an ISP that blocks port 25 and then only allows access to their SMTP server from a specific IP range. The result of this is a user who either has to use a different set of settings when they are not using their ISP to connect or to not be able to send e-mail at all! Because of this several services have setup with the sole intention of allowing a single SMTP server to be used in any circumstance, this is usually by listening on different ports to the standard SMTP ports. One such example is SMTP2Go.

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