Simple Mail Transfer Protocol (SMTP) is used to send email from a client to a server or between servers. It uses a store-and-forward process, in which the sender starts the transfer. An SMTP server can store a message until the receiving device comes online. At that point, it contacts the device and forwards the message. If all devices are online, the message is sent quickly.
Post Offıce Protocol version 3 (POP3) is a protocol that enables an email client application to retrieve email messages from a mailbox on a mail server. With POP3, the email messages wait in the mailbox on the server until the client retrieves them, either on a schedule or manually. Once the messages are retrieved and downloaded to the client, they are generally deleted from the server. The client then stores and works with the email messages locally.
POP3 and Multiple Computers
Because POP3 is designed by default to download messages to the local computer and delete them from the email server, it is not the best email protocol to use when users must access their email from multiple computers. When users access their email from multiple computers using POP3, they end up with their email messages downloaded and split among the computers they use instead of in one central location. If the POP3 server permits it, it is possible to conﬁgure options on the POP3 mail client to leave mail messages on the server. In this case, users will have to delete old messages from the server manually to avoid exceeding mailbox size limits.
Internet Mail Access Protocol version 4 (IMAP4) is a protocol that enables a client to retrieve messages from a mail server. With IMAP4, messages generally remain on the server while the client works with them as if they were local. IMAP4 enables users to search through messages by keywords and to choose which messages to download locally. Messages in the user’s mail-box can be marked with different status ﬂags, such as deleted or replied to. The messages and their status ﬂags stay in the mailbox until explicitly removed by the user. Unlike POP3, IMAP4 enables users to access folders other than their mailbox.
Tips: Because IMAP4 is designed to store messages on the server, it is easier for users to access their email messages—both new and saved—from multiple computers.
Tips: IMAP was developed at Stanford University in 1986