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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 ...
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 ...
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]
Within the Internet email system, a message transfer agent (MTA), [1] mail transfer agent, [2] or mail relay is software that transfers electronic mail messages from one computer to another using the Simple Mail Transfer Protocol. [3] In some contexts, the alternative names mail server, mail exchanger, or MX host are used to describe an MTA.
2 IGMP Internet Group Management Protocol: RFC 1112: 0x03 3 GGP Gateway-to-Gateway Protocol: RFC 823: 0x04 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 ...
[3] [8] [1] QUIC improves performance of connection-oriented web applications that before QUIC used Transmission Control Protocol (TCP). [2] [9] It does this by establishing a number of multiplexed connections between two endpoints using User Datagram Protocol (UDP), and is designed to obsolete TCP at the transport layer for many applications ...