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Email is submitted by a mail client (mail user agent, MUA) to a mail server (mail submission agent, MSA) using SMTP on TCP port 587. Most mailbox providers still allow submission on traditional port 25. The MSA delivers the mail to its mail transfer agent (MTA). Often, these two agents are instances of the same software launched with different ...
This is a list of TCP and UDP port numbers used by protocols for operation of network applications. The Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) only need one port for bidirectional traffic. TCP usually uses port numbers that match the services of the corresponding UDP implementations, if they exist, and vice versa.
To combat spam (including spam being sent unwittingly by a victim of a botnet) many ISPs and institutional networks restrict the ability to connect to remote MTAs on port 25. The accessibility of an MSA on port 587 [ 3 ] enables nomadic users (for example, those working on a laptop) to continue to send mail via their preferred submission ...
To cryptographically protect IMAP connections between the client and server, IMAPS on TCP port 993 can be used, which utilizes SSL/TLS. [2] [3] As of January 2018, TLS is the recommended mechanism. [20] Alternatively, STARTTLS can be used to encrypt the connection when connecting to port 143 after initially communicating over plaintext.
A standard SMTP service application listens on TCP port 25 for incoming requests. The second service is usually either the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP) which is used by email client applications on users' personal computers to fetch email messages from the server. The POP service listens on TCP port ...
Submission of new email from a mail client is via SMTP, typically on port 587 or 465, and is now generally restricted to servers the user has an account with-such as their ISP. This is for policy, not technical, reasons so that providers have some means of holding their users accountable for the generation of spam and other forms of email abuse ...
4 IP-in-IP IP in IP (encapsulation) RFC 2003: 0x05 5 ST Internet Stream Protocol: RFC 1190, RFC 1819: 0x06 6 TCP Transmission Control Protocol: RFC 793: 0x07 7 CBT Core-based trees: RFC 2189: 0x08 8 EGP Exterior Gateway Protocol: RFC 888: 0x09 9 IGP Interior gateway protocol (any private interior gateway, for example Cisco's IGRP) 0x0A 10 BBN ...
It can use a Transmission Control Protocol (TCP) transport like SMTP, but must not use port number 25, the well-known port for SMTP. Usage of port 24 ("any private mail system") is common among some mail server applications however. [2]