Difference between SMTP, POP3 and IMAP ?

Definition

SMTP or Simple Mail Transfer Protocol is used in the delivery of email from an email client to an email server. It is also used to deliver email from one server to another. The technology uses port 25. POP3 or Post Office Protocol is an email client that allows users to download email from email servers. A relatively simple protocol, POP3 has a relatively small feature set, and it doesn't really offer very much aside from download. POP3 works on the assumption that users download all the email from the server, before deleting them and disconnecting. POP3 uses port 110. IMAP or Internet Message Access Protocol is quite similar to POP3, in that it is also used to download email from a server. Aside from this function however, IMAP also allows users to keep their email on the server after accessing them. Because the protocol leaves the email on the server, IMAP has a lot more requirements with regard to disk space and CPU resources. IMAP uses port 143.

Advantages And Disadvantages 

The main drawback to SMTP is that it can be used only to send email and not to receive them. In addition, the use of SMTP is dependent on the system's network and/or ISP settings. The main advantage of POP3 on the other hand, is that the email can be stored on the server even after downloading it. This allows users to read their email at their own pace, even after cutting off Internet connection. The main drawback is that you may inadvertently delete spam email and even viruses in the process. The main advantage of IMAP is speed, since it requires only a relatively small amount of data to be passed.  Email messages will only be downloaded upon a specific request from the user. Other advantages include the ability to create email folders and/or mailboxes on the server itself, and user ability to delete messages.

In Use

SMTP is typically used by the Mail Transfer Agent or MTA for the delivery of email to the recipient's mail server. As mentioned previously, SMTP can only be used for sending emails. POP3 offers users a simple system for accessing mailboxes and downloading email messages. With POP3, the user has the option to download email from the server while retaining copies for later retrieval. IMAP requires the transfer of only a small amount of data, which means that it will work well even slow connections. In addition, IMAP offers a host of user access services as well.

• Post Office Protocol 3 (POP3) servers hold incoming e‑mail messages until you check your e‑mail, at which point they're transferred to your computer. POP3 is the most common account type for personal e‑mail. Messages are typically deleted from the server when you check your e‑mail.
• Internet Message Access Protocol (IMAP) servers let you work with e‑mail messages without downloading them to your computer first. You can preview, delete, and organize messages directly on the e‑mail server, and copies are stored on the server until you choose to delete them. IMAP is commonly used for business e‑mail accounts.
• Simple Mail Transfer Protocol (SMTP) servers handle the sending of your e‑mail messages to the Internet. The SMTP server handles outgoing e‑mail, and is used in conjunction with a POP3 or IMAP incoming e‑mail server.

Summary

SMTP

• Used in delivering email from one server to another
• Cannot be used for receiving emails

POP3

• Allows users to store email on the server even after downloading them
• Is pretty basic compared to the other protocols

IMAP

• Requires only small amount of data to be transferred 
• Will work even with slow Internet connections