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I found out here about how a postfix will get an MX record which could be several servers, and then try them on port 25 in order in order to ...
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- 01-29-2014 #1
- Join Date
- Mar 2013
How does a mail client know which server to connect to?
My next question is, then when the user wants to get their mail (POP or IMAP), how do they know which of those servers to connect to?
- 01-29-2014 #2
- Join Date
- Dec 2013
Sending mail and receiving mail are different functions. An email address is bound to an IP address and that is where the mail goes - unless someone is using their own server it will be an address provided by their service provider.
- 01-29-2014 #3
- 01-30-2014 #4
The DNS MX records tell the email world where to send email, so you might have, say, mx1.example.com pointed at your mail receiver for your domain, with backups through other MX records.
The mail clients will resolve their names as A (for ipv4), AAAA (for ipv6) or CNAME records in the DNS domain, and the name often set to be something memorable, along the lines of imap.example.com or mail.example.com. This is a good example of -why- the DNS system is so important.
This can, of course, be the same physical machine as the mail receiver - you just don't look up its name with the MX record. The reason that works is because these services run from different TCP/IP ports. An imap over SSL connection (quite common) would run on port 993. Regular pop3, I believe, uses 110.Linux user #126863 - see http://linuxcounter.net/