There are several SMTP ports but not all are created equal. The most frequently used is port 25 for SMTP relays, while port 587 is the default port for mail submission.
Port 25: The standard port
The first port, port 25, was implemented as the primary message transmission channel to help mitigate against attacks like man-in-the-middle. It came out of a 1982 request from the University of Southernn California to the Internet Engineering Task Force (IETF) who are the overlords of all protocols that control movement across the internet. Ironically, port 25 has since become favored by spammers because of its support for open relay.
SMTP port 25 continues to be used primarily for SMTP relaying. SMTP relaying is the transmission of email from email server to email server. In most cases, modern SMTP email clients (Microsoft Outlook, Mail, Thunderbird, etc.) shouldn't use this port. It is traditionally blocked by residential ISPs and Cloud Hosting Providers, to curb the amount of spam that is relayed from compromised computers or servers. Unless you're specifically managing a mail server, you should have no traffic traversing this port on your computer or server.
Port 587: The default port
In December of 1998, after port 25 started to get spammy, R. Gellens and J. Klensin submitted RFC 2476 in support of adding a new specification for internet email communications. The RFC proposed a split of the traditional message submission and message relay concept. The RFC defined that message submission should occur over port 587 to ensure new policy and security requirements don't interfere with the traditional relay traffic over message relay port 25. Port 587 is the default mail submission port. 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. All Mailgun Send customers should consider using port 587 as their default SMTP port unless you're explicitly blocked by your upstream network or hosting provider.
Port 465: The TLS port
The Internet Assigned Numbers Authority (IANA) has reassigned a new service to this port, and it should no longer be used for SMTP communications. Because port 465 was once recognized by IANA as valid, there may be legacy systems that are only capable of using this connection method. Typically, you will use this port only if your application demands it. A quick Google search, and you'll find many consumer Inbox Service Providers' (ISPs) articles that suggest port 465 as the recommended setup. However, we do not recommend it, as it is not RFC compliant.
Port 2525: The alternative port
This port is not endorsed by the IETF nor IANA. Instead, Mailgun provides it as an alternate port, which mirrors port 587, in the event the above ports are blocked. Because 2525 is a non-traditional high port number, it is typically allowed on consumer ISPs and Cloud Hosting providers, like Google Compute Engine. If you¡¦ve tried the above ports, but experience connectivity issues, try port 2525. This port also supports TLS encryption.
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