SMTP, POP3 and IMAP are TCP/IP protocols used for mail delivery if you have some sparetime, it is a good chance to learn more about what they are and how they differ from one another.
SMTP stands for Simple Mail Transfer Protocol, which is the protocol used to transport email messages from computer to another until they reach their destination.
SMTP is used between SMTP clients and SMTP mail servers, delivering an email to its destination.
SMTP still operates when SMTP mail servers forward the message to the next server, until it has been notified to the mail server for its final destination.
POP3 stands for Post Office Protocol version 3.
POP3 works as the main communication protocol or language, receiving emails from an email server or service provider. POP3 is used with some programs, for instance Thunderbird email, Microsoft Office Outlook, and other programs that use to communicate with other email servers. It can also be supported by an independent email program on other devices.
When POP3 is used, the email messages are moved to the computer where they are downloaded – . After the download is completed these emails will be deleted from the mail server on where they were stored.
Sometimes, POP3 is used to transfer emails from a different email provider. For example, your Outlook.com email access to Gmail can be implemented by configuring Gmail to use POP3 to retrieve and transfer (fetch and transfer) Outlook.com email into your Gmail account.
IMAP stands for Internet Message Access Protocol.
The name implies that IMAP is a protocol to access e-mails. It is different from POP3, a protocol to transfer (or move) message.
When IMAP is used by email programs to access email messages stored on a server, the messages are retained on the server unless they are moved or totally deleted. The copies of email messages can be downloaded, but basically, IMAP delivers the best which can be called a window or an overview for emails stored on the server.
While the copy of an email can be downloaded and enables offline access, the IMAP protocol works best when continuously connecting to the email server. Any changes on that server – for example, a new message arrives, an email is deleted or changed by a web interface or email program – will be notified via IMAP.
One of the IMAP’s strengths is multiple-simultaneous access. There are more than one computer or device can access the same collection of emails simultaneously. This technology is usually used by mobile devices, and even the web interface, e-mail management that can be accessed from multiple locations. It’s drawback is that it requires constant connection and that emails on the server will accumulates unless they are removed. This may lead to an excessive amount of data that is over the storage limit.