Since port 25 is blocked on my VPS, I need to configure Postfix to send emails to other ports such as 587 or 465. But it seems most incoming SMTP servers on the Internet such as outlook or gmail do not open port 587 or 465. They just use port 25 to receive emails. Is this a common practice?
I do not mean to use another SMTP server to relay emails (as talked in this post.)
Another question is if all(or most) SMTP servers that open port 587 need authentication? Port 587 is called submission port so I think it asks you for account information. Is there any incoming SMTP server that uses port 587 to receive emails and does not need account authentication?
Best Answer
465/587 are client submission ports, they are usually protected with authentication and transport layer security. Email clients, i.e. MUAs, use port 587 and 465, while mail servers, i.e. MTAs, use port 25.
Opportunistic TLS (Wikipedia)
The confusion probably comes from how SMTP used to communicate in the past. All communications (client to server, server to server) were made using TCP port 25. Nowadays it is uncommon for email clients to use port 25 for sending emails; also, many ISPs block outgoing port 25 on their client border to limit spam.
Point is, if a MTA is configured to use a different port than 25 then also the remote end needs to be configured to use that different port for the communication to be successful.
To support end-to-end TLS between MTAs, protocol extensions have to be implemented by both ends. See Increase email security with MTA-STS and TLS reporting (Google) and Extended SMTP (Wikipedia)