There are multiple prevalent standard email protocols:
Simple Mail Transfer Protocol (SMTP) is the standard protocol for sending email between:
For definitions of MTA and MUA, see Client-server connections in SMTP.
SMTP communications typically occur on TCP port number 25 and SMTPS generally occurs on TCP port number 465.
When an email user sends an email, their MUA uses SMTP to send the email to an MTA, which is often their email server. The MTA then uses SMTP to directly or indirectly deliver the email to the destination email server that hosts email for the recipient email user.
When an MTA connects to the destination email server, it determines whether the recipient exists on the destination email server. If the recipient email address is legitimate, then the MTA delivers the email to the email server, from which email users can then use a protocol such as POP3 or IMAP to retrieve the email. If the recipient email address does not exist, the MTA typically sends a separate email message to the sender, notifying them of delivery failure.
While the basic protocol of SMTP is simple, many SMTP servers support a number of protocol extensions for features such as authentication, encryption, multipart messages and attachments, and may be referred to as extended SMTP (ESMTP) servers.
FortiMail units can scan SMTP traffic for spam and viruses, and support several SMTP extensions.
Post Office Protocol version 3 (POP3) is a standard protocol used by email clients to retrieve email that has been delivered to and stored on an email server.
POP3 communications typically occur on TCP port number 110.
Unlike IMAP, after a POP3 client downloads an email to the email user’s computer, a copy of the email usually does not remain on the email server’s hard disk. The advantage of this is that it frees hard disk space on the server. The disadvantage of this is that downloaded email usually resides on only one personal computer. Unless all of their POP3 clients are always configured to leave copies of email on the server, email users who use multiple computers to view email, such as both a desktop and laptop, will not be able to view from one computer any of the email previously downloaded to another computer.
FortiMail units do not scan POP3 traffic for spam and viruses.
Internet Message Access Protocol (IMAP) is a standard protocol used by email clients to retrieve email that has been delivered to and stored on an email server.
IMAP communications typically occur on TCP port number 143.
Unless configured for offline availability, IMAP clients typically initially download only the message header. They download the message body and attachments only when the email user selects to read the email.
Unlike POP3, when an IMAP client downloads an email to the email user’s computer, a copy of the email remains on the email server’s hard disk. The advantage of this is that it enables email users to view email from more than one computer. This is especially useful in situations where more than one person may need to view an inbox, such where all members of a department monitor a collective inbox. The disadvantage of this is that, unless email users delete email, IMAP may more rapidly consume the server’s hard disk space.
FortiMail units do not scan IMAP traffic for spam and viruses, but may use IMAP when operating in server mode, when an email user retrieves their email.
HTTP and HTTPS
Secured and non-secured HyperText Transfer Protocols (HTTP/HTTPS), while not strictly for the transport of email, are often used by webmail applications to view email that is stored remotely.
HTTP communications typically occur on TCP port number 80; HTTPS communications typically occur on TCP port number 443.
FortiMail units do not scan HTTP or HTTPS traffic for spam or viruses, but use them to display quarantines and, if the FortiMail unit is operating in server mode, FortiMail webmail.