POP3 & IMAP protocols are used to retrieve emails from and email server on a mobile, tablet or PC. Whereas SMTP is a protocol used for sending emails to email servers
POP3 (Post Office Protocol)
POP3 is abbreviation which stands for Post Office Protocol 3. It is a very simple protocol in the sense that it simply downloads anything in “Inbox” folder from your email server. It is worth remembering that it does not download any other content/folder from your email server except what is lying in “Inbox” folder. POP3 protocol generally uses port 110 for communication.
POP3 protocol has no synchronization with the server which means that any messages marked as read/deleted/replied to in the mail application will not be available to see on server. It is also important to understand that once an email is downloaded using POP3 protocol on a device it is automatically deleted from the server.
Advantage with POP3 is that it saves storage space on the email server by automatically deleting the emails which have been downloaded. Another advantage using POP3 is that once an email has been downloaded one does not need to be connected to internet to read it. The disadvantage of POP3 is that since there is no copy on server in case the local device crashes all data is lost.
IMAP (Internet Message Access Protocol)
IMAP in computer world stands for Internet Message Access Protocol. It allows users to view your emails from multiple devices on the server. It synchronizes all folders on server and caches a copy of email on your local device. This means all messages that have been read/deleted/replied to will be available both on the server and mail application. Let’s take an example to understand it better. If any email is deleted form email application such as “Microsoft Outlook Express” due to automatic syncing by IMAP the email will be deleted from server as well.
IMAP generally uses port 143 for standard communication with servers.
Major advantage of using an IMAP server is that it helps when a user needs to retrieve his emails from different devices and he wants to view all folders including sent/deleted and custom folders.
An internet connection is always required to view emails on IMAP as it does not download copies of emails on local device and it only caches them.
SMTP(Simple Mail Transfer Protocol)
SMTP stands for Simple Mail Transfer Protocol which enables users to send emails to an email server on the internet. All email client applications such as Microsoft Outlook Express or Thunderbird support SMTP protocol. This protocol runs over TCP/IP protocol and uses TCP port number 25 for standard communication. Some private web email servers such as Gmail use non-standard TCP port 465 for SMTP communications.
It is a very old protocol which have seen some improvements over the last few decades and still has limitation as it queues up emails on receiving server. An alternative protocol that we know of for outbound transmission of email is called X.400. It is getting popular in Europe and Canada. Extended Simple Mail Transfer Protocol (ESMTP) is also getting lot of support from many mail servers as it allows multimedia files to be delivered as emails.
