Resources and Info

How Email Works

There is much more to email than meets the eye. We provide here, some background information about email and how it works, and a simple step-by-step diagram showing how an email message is sent and received.

What is an email client?

Email servers are used to process, store and send, and receive email messages. There are different types of email servers, such as POP3, SMTP, and IMAP servers. The most popular email servers are POP3 (incoming mail servers for your incoming mail) and SMTP (outgoing mail servers for your outgoing mail).

What is SMTP or outgoing mail server?

When you send an email from your email program, if you have an Internet connection with Comentum, the email first arrives at our SMTP (outgoing mail server), then our SMTP server will check for the validity of both source and destination email addresses. Then it will send your email to the proper destination. You use your Internet conection's email server when you send mail.

What is POP3 or incoming mail server?

When someone sends you an email message, it arrives at our POP3 server and it waits for you to check your email. When you check your email, your email program accesses our POP3 servers and downloads your waiting messages from our POP3 server to your computer.

What is Web Mail?

Sending and receiving email messages using a web browser is referred to as "Web Mail." Using Web Mail is ideal when traveling or using another person's computer or an outside computer, such as at the library or at a tradeshow. You can access Comentum's Web Mail as long as you have a browser and an Internet connection. You can always use Comentum's Web Mail in conjunction with your usual email program, such as Outlook Express.

What is the difference between Web Mail and Client Email (such as Outlook Express)?

Client email, such as Outlook Express will ordinarily download and save your messages to your computer, and then it deletes your email messages from our servers. Comentum's Web Mail will not download and save your messages to your computer; it will leave your messages on our servers, unless you manually delete them.

How does email travel when I receive or send an email message?

When Lisa sends Bob an email message (Figure B), the following is a simplified step-by-step of how it travels:

  1. Lisa opens her email program (Outlook Express), uses her email account,, and writes an email message to Her Outlook Express sends the message through TCP port 25 of Lisa's Internet connection, then to the other ISP's SMTP server (still on TCP port 25).
  2. The other ISP's SMTP server finds the destination's SMTP server (in this case, Comentum) and sends the message out to Comentum's SMTP server (through TCP port 25 of her Internet connection).
  3. Comentum's SMTP server sends the message to Comentum's POP3 server where the message waits for Bob to pick it up.
  4. Bob opens his email program (Outlook Express) and when he clicks Send/Recv button, his Outlook Express sends a request through Bob's Internet connection to Comentum's POP3 server (through TCP port 110) and downloads Bob's message to Bob's computer where he sees Lisa's message.

The same process happens in reverse, when Bob sends Lisa an email message (Figure A on the front).

* There are other network elements that are necessary to make this process happen, such as: Comentum's routers, Comentum's DNS servers and Comentum's Internet connection, and other ISP's routers, DNS servers and Internet connection. We left those elements out and concentrated on email clients and email servers.

** Bob may use a different ISP for his Internet connection and to host his domain, website and email ( uses the same Comentum mail servers described in Figure A and B.

For Information Contact:
Comentum Corp
6222 Ferris Sq.
San Diego, CA 92121
Phone: 619-990-1212

Hours: Mon. - Fri., 9 a.m. - 5 p.m. PST

Contact Us »

Mobile App Development

iPhone, Google Android, BlackBerry, Windows Mobile
In-house development team.
San Diego, CA, USA.

Learn More

Web App Estimator

Select and unselect modules such CMS or eCommerce for your web application and watch the cost update in real time.

Try It »