POP3 & SMTP Protocols: Answers to Common Questions

[fisico30]

Hello Forum,

in order to configure our email to send and receive email we need need to use an email client program that supports the Post Office Protocol (POP3) and Simple Mail Transport Protocol (smtp) standards. POP is a protocol for retrieving mail from the ISP mail server while smtp is used for sending mail (mailman).

The mail servers have an IP address. Port 110 and port 25 are the two logical ports on the same mail server. Do these ports represent two different parts of the memory storage of the server?

Sometimes, when our email is not working correctly we are told to make changes to our client configuration...
POP3 is also called an incoming Mail Server, like IMAP and HTTP. SMTP is called the outgoing Mail Server. Are the incoming and outgoing mail servers the same physical computer or two different ones?

Do ISP have multiple incoming and multiple outgoing mail servers?

Can there be more than one incoming mail server for one email account?

thanks
fisico30

 

[MATLABdude]

Hello Forum,

in order to configure our email to send and receive email we need need to use an email client program that supports the Post Office Protocol (POP3) and Simple Mail Transport Protocol (smtp) standards. POP is a protocol for retrieving mail from the ISP mail server while smtp is used for sending mail (mailman).

The mail servers have an IP address. Port 110 and port 25 are the two logical ports on the same mail server. Do these ports represent two different parts of the memory storage of the server?

Sometimes, when our email is not working correctly we are told to make changes to our client configuration...
POP3 is also called an incoming Mail Server, like IMAP and HTTP. SMTP is called the outgoing Mail Server. Are the incoming and outgoing mail servers the same physical computer or two different ones?

Do ISP have multiple incoming and multiple outgoing mail servers?

Can there be more than one incoming mail server for one email account?

thanks
fisico30

SMTP, POP and IMAP are protocols followed by pieces of software that might or might not run on the same physical computer (for instance, my friend's old setup which hosted his company's e-mail and web, his FTP and even an internet radio station--it was a really small scale operation). My understanding of ports is that they're like internal mail addresses (e.g. "packet for mail room!")
http://en.wikipedia.org/wiki/List_of_mail_servers

Most larger ISPs / companies / schools / organizations probably have quite a few computers hosting each of these services, each accessing the data stored on a central file server (or few). I don't really know the details of such things, but large-scale internet operations are quite sophisticated and complex. For instance, Wikipedia elaborates on their operations:
http://en.wikipedia.org/wiki/Wikipedia#Software_and_hardware

 

[jtbell]

My understanding of ports is that they're like internal mail addresses (e.g. "packet for mail room!")[/url]

Pretty much so. Each TCP/IP packet has a field which contains a "port number." The low-level TCP/IP software on a server looks at that number and routes the packet to a corresponding application, as listed in a table somewhere. On many Unix or Unix-like systems, this table is in the file /etc/services. Here's part of that file on my Mac OS X system, for ports 20-25:

ftp-data         20/udp     # File Transfer [Default Data]
ftp-data         20/tcp     # File Transfer [Default Data]
ftp              21/udp     # File Transfer [Control]
ftp              21/tcp     # File Transfer [Control]
#                          Jon Postel <postel@isi.edu>
ssh              22/udp     # SSH Remote Login Protocol
ssh              22/tcp     # SSH Remote Login Protocol
#                          Tatu Ylonen <ylo@cs.hut.fi>
telnet           23/udp     # Telnet
telnet           23/tcp     # Telnet
#                          Jon Postel <postel@isi.edu>
                 24/udp     # any private mail system
                 24/tcp     # any private mail system
#                          Rick Adams <rick@UUNET.UU.NET>
smtp             25/udp     # Simple Mail Transfer
smtp             25/tcp     # Simple Mail Transfer

 

[MATLABdude]

That's weird: what's with all the names and e-mail addresses?

 

[jhae2.718]

cat /etc/services is rather interesting...

 

[jtbell]

what's with all the names and e-mail addresses?

Those are comments for human readers, signaled by the # mark at the beginning of those lines. The TCP/IP software ignores them.

I suspect those are the people who were responsible for making those port number assignments in the first place. For example, a Google search for "Tatu Ylonen" reveals that he was one of the designers of the SSH (Secure Shell) protocol. And Jon Postel wrote RFC 318 which first defined the telnet protocol.

This is the first time I remember seeing those comments included in an /etc/services file.

 

[jhae2.718]

I personally prefer RFC 1149.

 

[I like Serena]

The mail servers have an IP address. Port 110 and port 25 are the two logical ports on the same mail server. Do these ports represent two different parts of the memory storage of the server?

The ports represent so called "sockets" and they provide a place on a computer to "plug in".
There is no memory associated with a port, but the information sent and received is handled by the TCP/IP kernel driver and queued in memory.
Port 25 is the SMTP port to which a server listens for messages that people want to send.
Port 110 is the POP3 port to which a server listens for requests of people to view and manipulate their mailbox.

Sometimes, when our email is not working correctly we are told to make changes to our client configuration...
POP3 is also called an incoming Mail Server, like IMAP and HTTP. SMTP is called the outgoing Mail Server. Are the incoming and outgoing mail servers the same physical computer or two different ones?

Incoming and outgoing mail servers can be the same or different, it does not matter.

An SMTP server acts like a router, redirecting mail to another SMTP server if necessary. The final SMTP server "drops" the email in the appropriate mailbox.
This means you can send your mail to any SMTP server. It's only recently that mail servers of major internet providers only accept mail from their own clients.

A POP3/IMAP server provides the service to access your mailbox, to list your messages, to read them, and to delete them. POP3 and IMAP are two different protocols to do the same thing, although IMAP has more advanced functionality.

A HTTP server typically shows your email in HTML pages, but redirects all requests to a POP3 or IMAP server.

In all cases the request messages that are sent are simple plain text messages. Response messages are also plain text, although often binary data will be encapsulated in those messages.