Email protocol is a method by which a communication channel is established between two computers and email is transferred between them. When an email is transferred, a mail server and two computers are involved. One computer sends the mail and the other one receives it. The mail server stores the mail and lets the receiving device access it and download it if needed. There are four different mail protocols. These protocols differ in the way by which they establish connections and allow user access to emails.
Usage in Various Email Applications
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Various email applications exist. Microsoft Outlook is one of the most popular commercial applications. Lotus Notes is another particularly common one. These applications can support various email protocols. Different ports are used for different mail protocols. Microsoft Outlook can support mail servers compatible with IMAP and POP3 protocols. UNIX machines and simple mail applications support SMTP protocol. Following are the different email protocols.
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POP3 (Post Office Protocol 3)
This is a simple, standardized protocol that allows users to access their mailboxes on the Internet and download messages to their computers. The simple design of POP3 allows casual email users who have a temporary Internet connection (dial-up access) to access emails. They can read their emails, draft new emails or reply to emails while they are offline, and can send these emails when they are back online. Yahoo! Mail (mail.yahoo.com) is an example of a mail server that uses POP3 protocol.
Email clients like Microsoft Outlook may be used to access emails and download them from mail.yahoo.com. Users may also access them on web browsers like Internet Explorer.
IMAP (Internet Message Access Protocol)
This is a standard protocol used for email transfer by users. Like POP3, it also supports both online and offline modes of email access. The email message is downloaded to the user's machine only when a specific request is made to read it. Users can download mails to their computers while keeping a copy on the server. The mails on the server are the primary copy and anything changed on the local machine is updated by what is on the server.
IMAP provides powerful search capability for savvy email users to search for emails in server. This is useful, as the mails need not be downloaded--saving time for users.
Users can also create new mailboxes which are shown as folders on the server, and move messages between folders. This feature allows access to shared and public folders.
Gmail is an example of a mail server which uses IMAP protocol. Email clients like Microsoft Outlook may be used to access these emails directly on the server and the local machine after downloading. Again, users may use IE to access these emails.
SMTP (Simple Mail Transfer Protocol)
As the name suggests, SMTP is a simple, text-based protocol that works best when devices are interconnected to each other. However, SMTP protocol can only be used to send emails. Unlike POP3 and IMAP protocols, SMTP does not provide the functionality to the users to retrieve emails from the server. This limits the use of SMTP to some extent. To overcome this problem, SMTP provides a feature to queue mails on a server so that the messages bound for the receiving system could be delivered.
One of the main issues with SMTP is the lack of sender email authentication. SMTP lacks security features too and thus users get spam emails.
UNIX systems and simple applications like "mailsend" make use of SMTP protocol. Commercial applications don't use SMTP to a great extent.
HTTP (Hyper text transfer protocol)
Even though HTTP is not used exclusively for mail transfer, it still plays a vital part for users who use Internet browsers for accessing their mails (both for sending and receiving).
Hotmail and Yahoo! use HTTP protocol for accessing emails through the Internet.