04-16-2013 09:21 PM
SSL = Secure Socket layer
POP = Post Office Protocol
IMAP = Internet Message Access Protocol
SMTP = Simple Mail Transport Protocol
What are ports?
Ports are not physical, they are more like a concept. They are like a label or designation. Think about it like this: Let's say that a friend says that they will leave a message for you at their apartment building. There are 20 apartments adn there are 20 hooks at the entryway where you can hang messages. The friend lives in apartment 12. He tells you that he will leave the message for you on hook 8. You ask why the message will not be left on hook 12. The friend says, "Because I am going to leave it on hook 8. If pull a message from any other hook, it will be the wrong one." So, essentially the two parties have decided that hook 8 will be designated as their point of communication. It is arbitrary. There is no rhyme or reason to it. It just is. So, let's say your email app will be setup to "listen" for messages at port 995. The app will not accept messages designated for any other "port". Let's say that an email server for outgoing messages is "listening" at port 465. If you email app sends messges to any other port, the email server will ignore them and the messages will never be sent. Hope that the explanation makes sense. If I said something that wasn't quite correct, feel free to correct me.
04-17-2013 01:41 PM
An important thing to remember about Ports.
They often vary across email providers.
While there are some 'default' and 'suggested' port numbers to use, often Email Providers will use whatever they choose.
So, when setting up email accounts manually, always try to find settings for your exact email provider. Usually, the home page of the email provider will have a link to settings.
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04-17-2013 11:02 PM
Port numbers below 1024 can not be used willy-nilly. Those ports are assigned specific use by ICANN (or similar committee). Port 25, for example, is always used for classic SMTP, and may still be used between ISP servers. It used to be that one could send email to anyone using any SMTP server, not just the one belonging to your ISP -- SMTP did not require any user authentication. That led to spammers sending mail on any server the could connect to.
The first step in tightening SMTP was to stop open relaying. A server would now only accept a message if it came from an inside node (your computer connected directly to you ISP, having an IP address in that domain) OR if the mail is coming from outside, it has to be addressed to someone on the inside. Otherwise the message is rejected.
Then came WiFi and the like. It would have been a hassle to have to change your mail server settings every time you changed hotspot. Enter the alternate "submission" port. Being a new port assignment the protocol could be tweaked to require a user login (unlike port 25 SMTP)
04-17-2013 11:11 PM
The third concept was concern as SMTP is a plain text protocol, even when using the alternate submission port. So SSL/TLS with its port number were added to session encrypt the message.
POP3 was always an authenticated protocol (one had to prove the mail was their's to retrieve). However, the plain text situation also applied, so a new port for SSL/TLS was assigned.
IMAP is a whole 'nother can of worms.