On 2020-05-29 Matteo Cazzador wrote: > Hi, excuse a question, during the comunication between 2 mail servers > there is a phase of according about che port to use to comunicate? > > There is a checking about port 587 or 25 during these comunication? > or two servers only use 25 port by default? > > Sorry for the level of the question I ask you.
No, mail servers don't negotiate which port to communicate on. Port 587 (submission) is the port for MUA to MTA communication. That port should be used when a mail client (MUA, mail user agent) is initially sending/submitting mail to its upstream mail server/relay. Also, submission normally requires both encryption (TLS) and authentication. Port 25 (smtp) is the starndard port for MTA to MTA communication (MTA means mail transfer agent, vulgo mail server). Unless the sending MTA is configured to use a different port for a specific next hop that port will be used. Of course an MTA can also use submission to send to a next-hop MTA if the latter supports that (since the sending MTA is acting as a client there), but you would need to specifically configure that on the sender. Regards Ansgar Wiechers -- "Abstractions save us time working, but they don't save us time learning." --Joel Spolsky
