SMTP, or Simple Mail Transfer Protocol, is the accepted global standard for sending email messages across networks. How SMTP relays work is a foundational aspect of how email marketing works, so having a solid understanding of it is recommended for any email marketing professional.
Specifically, when configuring a site or mail client for outgoing mail, you need to understand the role that SMTP ports play in email communication.
You should also be aware of the most widely used official SMTP ports, formerly recognized ports, and alternatives that have more niche, situational use cases. SMTP port 2525 is an example of such an alternative.
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SMTP, ports, and functions
To fully describe the purpose of SMTP port 2525, let’s first outline the role of ports within the Simple Mail Transfer Protocol.
The SMTP process is a technical one, but for the sake of simplicity it can be broken down into three steps, as below:
Step one: A sender authors an email on their device, fills in the address of the recipients, and clicks the send button. This prompts the mail client to commence communication with the outbound SMTP server that handles the sender’s outgoing mail. This process of directing messages to an outgoing SMTP server is known as SMTP submission.
Step two: The outgoing SMTP server performs a DNS (Domain Name Server) lookup using the recipient’s email address and retrieves the relevant IP address. At this point, the outgoing server communicates with the inbound server of the recipient and transmits the message to it. This interaction between two mail servers is called SMTP relay.
Step three: The inbound server accepts the email message, authenticates it, and routes it to the mailbox of the recipient.
For the Simple Mail Transfer Protocol to work, an IP address and a port number are required. Different SMTP ports have different functions, with some used for SMTP submission and some for relay.
Port 2525 is typically used for email submission.
Port 2525 and Port 587
At present, port 587 is considered the universal standard for email submission. This means that port 2525 is not technically considered an official SMTP port, as it is not recognized by the IANA (Internet Assigned Numbers Authority) or the IETF (Internet Engineering Task Force).
Despite its unofficial status, port 2525 sees widespread use as an alternative to SMTP port 587, and is supported by major ESPs (Email Service Providers). As is the case with port 587, port 2525 supports TLS (Transport Layer Security) to provide secure email submission.
Ports 25 and 465 have both been used for SMTP submission in the past. However, port 25 is now primarily used for SMTP relay, while port 465 has been deprecated.
Although some legacy systems may still support these, neither is recommended for email submission. Due to the secure nature of port 2525, it has become the default alternative port for submission when users experience issues with port 587.
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Port 2525 is an alternative SMTP port used for email submission, the process of transmitting mail to an outbound mail server. Although it is not officially recognized by the IETF or the IANA, port 2525 is the most commonly used alternative for SMTP submission in cases where port 587 is not working.
Though port 2525 is not the only port available for submission, its use of TLS encryption separates it from other potential options, such as port 25 or port 465. For this reason, port 2525 is generally supported by major ESPs and ISPs (Internet Service Providers).
In summary, port 587 is the recommended option for email submission. However, in cases where users experience blockages or connectivity issues with port 587, port 2525 offers a useful second option as it mirrors the functions of port 587 while providing a high level of security for users.