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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 ...
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.
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.
An application binds a socket to its endpoint of data transmission, which is a combination of an IP address and a port. In this way, UDP provides application multiplexing. A port is a software structure that is identified by the port number, a 16-bit integer value, allowing for port numbers between 0 and 65535. Port 0 is reserved but is a ...
This is a list of Simple Mail Transfer Protocol (SMTP) response status codes. Status codes are issued by a server in response to a client's request made to the server. Unless otherwise stated, all status codes described here is part of the current SMTP standard, RFC 5321. The message phrases shown are typical, but any human-readable alternative ...