Understanding SMTP Ports and How to Pick the Best One
Email delivery depends on Simple Mail Transfer Protocol (SMTP). Selecting the incorrect SMTP port will prevent your emails from reaching their destination. This comprehensive guide serves to teach you everything you need to know when it comes to SMTP ports for your email infrastructure.
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We’ll start with the basics and build up from there:
By the end, you’ll have in-depth knowledge of SMTP ports to make informed decisions when setting up business email.
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So, what is SMTP and its purpose? SMTP enables the sending and transportation of email messages between mail servers over the internet. They form a crucial backbone of all email delivery infrastructure. An SMTP port is the endpoint over which email communication happens using the SMTP protocol.
Every Internet connection requires two main parameters:
The port number tells the server which application to route the traffic to. For example, web traffic goes to port 80, while email routing uses port 25. Using an SMTP API service can streamline the process of automating email sending. However, general understanding of SMTP ports is necessary to make proper connections.
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Without an open SMTP port, you cannot send and receive emails, period. Most email providers and servers block unauthorized network traffic. So, you need valid SMTP credentials to identify yourself and get access.
The SMTP port, along with the SMTP server detail, allows your emails to be accepted for delivery. Without them, your email client or application cannot hand over emails to the mail server for transportation. Using email testing software ensures that your SMTP setup is working correctly and that emails will be delivered without issues.
This means that if you are setting up a business email client like Outlook or if you are going to configure an app for sending notifications, an open port of SMTP is needed.
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There are a few standard SMTP ports that see widespread use (aka well-known ports):
Port 25 – The default SMTP port for mail routing
Port 465 – Deprecated implicit SSL/TLS encryption port
Port 587 – Modern SMTP channel for secure mail submission
In addition, there are some extras:
Port 2525 – Common unregistered alternative to port 25 with TLS support
Port 2465 – Assigned for testing/troubleshooting purposes
The most common ports used by SMTP are 25, 587 and 2525, so we will discuss those three in this guide. Each has some pros and cons depending on which use case you have.
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The SMTP ports 25, 587, and 2525 appear to be interchangeable on the surface. However, one important thing you should know is that there are some key differences.
Native SSL/TLS encryption is not initially supported on port 25. Thus, it is an insecure channel through which data may be exposed to interception or manipulation.
Ports 587 and 2525 both support modern STARTTLS and TLS encryption standards for secure data transmission.
Port 25 is the officially designated SMTP port as per the original RFC. So, it’s still the default port that all mail systems attempt to connect to first. However, due to abuse, some providers block connections to this port.
Meanwhile, ports 587 and 2525 serve as more specialized alternatives.
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Port 25 handles all types of SMTP traffic, including initial message submission and further mail relaying.
Port 587 is meant for only the initial secure SMTP submission of messages from mail clients to servers. Port 25 handles subsequent server-to-server relaying.
So, in essence, port 587 separates submission from relay for added security.
Being the SMTP default port, port 25 has near-universal compatibility with email servers and systems.
Ports 587 and 2525 have very high adoption but still face occasional issues with restrictive networks.
Many ISPs and networks deliberately block port 25 traffic to curb spam and abuse. Ports 587 and 2525 are favored for requiring SSL and login credentials upfront.
As you can see, SMTP ports have their nuances even though email delivery relies on the same SMTP protocol in each case. Factoring in these aspects is important when deciding the right port.
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Your ideal SMTP port depends on your specific email port infrastructure and needs:
For sending high volumes of email like marketing campaigns, newsletters etc., the best ports are:
They both offer native SMTP TLS port encryption over security. Fewer blocks are seen by restrictive networks over port 2525 than port 587.
Whichever port you use, authentication and SPF/DKIM are highly recommended for the best deliverability.
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If you need to send emails from a website, like order confirmations and password resets, go with:
These ports force encryption and check credentials to minimize spam complaints.
For the highest compatibility across hosts, make sure to offer both as alternatives.
To configure company email for under 50 employees, stick to port 587. It strikes the right balance between security and hassle-free compatibility for small operations. Make sure to use an SMTP provider that offers this port.
Large organizations with their own on-premise Exchange/Office 365 servers should use port 25. Port 25 provides enterprises with the wider operating range and network flexibility they require.
Port 25 abuse is locked down with strict security policies, DLP filters, and monitoring.
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The SMTP port itself offers no native security or encryption. To harden your email delivery setup:
STARTTLS and implicit TLS immediately encrypt connections over ports 587 and 2525.
Mandate SMTP auth using strong credentials before allowing any mail transfers.
Adding Sender Policy Framework and DomainKeys Identified Mail validates your right to send mail.
Watch port activity to detect any suspicious anomalies rapidly.
Only allow the bare minimum servers and IP ranges to connect to your SMTP infrastructure.
Following these best practices closes major security gaps regardless of your port.
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During your email setup, you may face issues like server timeouts or rejected connections. Some common causes related to ports include:
Many residential ISPs block outbound traffic on port 25. Verify if your network permits port 25 access first.
If blocked, use port 587 or 2525 instead, along with SMTP authentication.
A connection refusal error means your SMTP server is not listening on that specific port. Double-check which ports your mail server has opened.
Then, verify if your local network blocks that SMTP port through firewall policies or proxy filtering rules.
An outgoing email stuck in your local outbox means your SMTP client cannot communicate successfully with your configured mail server.
If you have an alternative port configured, try switching from port 587 to 2525 or vice versa. Also, temporarily disable any VPN or firewall for diagnosis.
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Rather than managing your own SMTP server, you can use a dedicated third-party SMTP service behind the scenes to send emails. The advantages are:
For business-grade reliability, UniOne is an excellent SMTP provider option. Some key benefits are:
Other than being the very best alternative to its natural cousin, UniOne also has a pristine sender reputation, ensuring that your mail gets to the inbox instead of your spam folder.
Real-time delivery of email status is tracked using UniOne, and you will get immediate alerts of bounce or any delivery issues.
The message is passed through custom retry logic, intelligent routing, and feedback loops to ensure that each message makes it to the recipient’s inbox.
UniOne’s high-capacity infrastructure maintains fast delivery speeds even at scale.
Send mail over port 25, 2525, or 587 based on your needs. Port 465 is not supported, however.
With UniOne handling email delivery and ports behind the scenes, you can focus on business goals rather than email infrastructure.
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Knowing the differences between SMTP ports will help you understand how to set up your email delivery pipeline.
Ports 587 and 2525 are more optimized for SMTP secure communications but come at the price of not being as wide legacy compatible as port 25.
Additionally, doing things the right way, i.e., encrypting, authenticating, and monitoring, makes your SMTP hygiene as secure as possible.
When it comes to maximum deliverability, inbox placement, and reduced overhead, the best thing to do is to use a purpose-built SMTP provider like UniOne.
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