SMTP uses port 25 and it has been allocated for both UDP and TCP. I have never seen use of UDP in SMTP. In TCP, a packet is sent and checked on the other end. If the checksum says the packet is corrupt, it is dropped and a request is sent to retransmit it. It is used for situations that every single bit of information must be accurate.
TCP: Simple Mail Transfer Protocol (SMTP) 5321: smtp: Mail (sending email); iCloud Mail ...
Normal usage: SMTP (outbound email) traffic. This is unencrypted – for the encrypted version, see 465/TCP. Inbound port 25 traffic may be blocked by one’s ISP; this policy came in response to spammers using unprotected SMTP servers to relay spam. Port 53/TCP
587. tcp. smtp. Outgoing SMTP Mail port (TLS/Start TLS Port) - used by various mail servers for relaying outgoing mail as a modern alternative to port 25. Gmail, Apple MobileMe Mail, Yahoo SMTP server, etc. all use this port. See [ RFC2476] IANA registered for: Message Submission (TCP/UDP) SG. 587.
TCP and UDP; 23: Telnet: Port used by Telnet to remotely connect to a workstation or ...
The protocol version in common use today has extensible structure with various extensions for authentication, encryption, binary data transfer, and internationalized email addresses. SMTP servers commonly use the Transmission Control Protocol on port number 25 (for plaintext) and 587 (for encrypted communications).
When an email client or outgoing server is submitting an email to be routed by a proper mail server, it should always use SMTP port 587 as the default port. This port, coupled with TLS encryption, will ensure that email is submitted securely and following the guidelines set out by the IETF.
Does email use TCP or UDP? Email Protocols All three use TCP, and the last two are used for accessing electronic mailboxes. Special records stored in DNS servers play a role as well, using UDP. Is port 587 a SSL? msa (port 587) Ports 465 and 587 are intended for email client to email server communication – sending out email using SMTP protocol.