Port 587 is another SMTP submission port that’s supported by the vast majority of servers, and will definitely reduce the rate of rejected messages. That is particularly helpful if you’re sending bulk or mass emails (e.g. newsletters): in this case you should also get a dedicated SMTP server to maximize your deliverability and handle unlimited messages.
Port 587 is the default mail submission port that supports all types of SMTP data transmission. Supported by the majority of servers, this port is a popular choice for handling mail submissions. Combined with TLS encryption, SMTP port 587 ensures your emails are secure and delivered safely.
Technically both the ports are same and capable enough to support all sorts of SMTP related data transmission. It’s the addon security layer which makes port 587 as the preferred port for the new generation applications. It is the one recommended for mail submissions instead of port 25 as per RFC 2476. But even if the mail server supports it, it may or may not be open for mail submissions.
Port 587, coupled with TLS encryption, ensures that email is submitted securely and following the guidelines set out by the IETF. All Mailgun customers should consider using port 587 as their default SMTP port unless you're explicitly blocked by your upstream network or hosting provider. Wait, what about POP and IMAP?
Port 587 is the default port for SMTP submission on the modern web. While you can use other ports for submission (more on those next), you should always start with port 587 as the default and only use a different port if circumstances dictate (like your host blocking port 587 for some reason).
Port 2525: A common alternate SMTP port. Port 2525 is not an official SMTP port, and it is not sanctioned by the IETF nor IANA. However, SparkPost and many other email service providers support the use of port 2525 as an alternative to port 587 for SMTP, in the event the above ports are blocked.
SMTP Port 587. SMTP Port 587 is the default port for email submission and used by a lot of parties. If you don’t know the default port of the remote email server the best choice to start check for submission is used the SMTP port 587. Port 587 is provided as a TCP port. Also, port 597 provides a secure email submission by using TLS encryption.
Tl;dr Port 587 is the default port for SMTP message submission. Port 587 has always been the default port for message submission. The confusion around port 465 and port 587 stems back to 1997 when a standard for encrypted transit was being discussed. Ultimately, StartTLS was the protocol chosen.