Why VPS providers block outgoing email ports by default due to spam

Email remains a core method of internet communication. Its original technical design predates many modern safeguards, making it simple to send - but also easily abused for spam. This article covers why VPS (virtual private server) hosting companies commonly restrict outgoing ports related to email, like 25, 465, and 587, and what users can do about it.

How email spam developed

The Simple Mail Transfer Protocol (SMTP) forms the backbone of email and was designed decades ago with minimal barriers to sending messages. Because of this, automated tools can send massive volumes of unsolicited emails with little technical difficulty or computational cost.

Historically, this resulted in abuse by senders distributing unwanted advertising, scams, and phishing attempts to every address they could find. This, in turn, led developers and administrators to invent anti-spam mechanisms to preserve the reliability and usability of email.

Ironically, the first-ever email spam was sent by an MIT sysadmin of CTSS in 1971 to hundreds of ARPANET users warning about email spam that began with the sentence:

“There is no way to peace. Peace is the way.”

Spam blocklists and mailserver reputation

To fight junk mail, the industry relies on publicly available blocklists. If a mailserver’s IP address is reported or detected as sending spam, it is added to these lists and may be denied by receiving servers. Some key features:

• Reputation systems: Large email providers (Gmail, Outlook, iCloud Mail, Yahoo, etc.) maintain internal tracking of sender reputation, making delivery difficult for servers with histories of abuse. Metrics like how many emails sent by you are opened (called the Click Through Rate (CSR)) among others are taken in consideration.
• Blocklist checks: Publically available spam blocklists maintain a list of IPs that send out unwanted emails in large amounts. You can use free online services to check if your server’s IP is on these lists. They usually include options to appeal to the block.

For example, after obtaining an IP address from your hosting provider, you can check your mailserver’s IP reputation with the popular lists such as Spamhaus or MXToolBox’s free tool to do a lookup against major blocklists and appeal to them directly.

VPS providers and port restrictions

Due to the ease of obtaining new IPs and cheap temporary servers, spammers have historically used them to launch mass email campaigns. Nowadays, to limit abuse, most VPS providers block outgoing traffic on SMTP standard ports (such as 25, 465) by default.

If your server needs to send legitimate email, you can contact your VPS provider to request port unblocking.

You can also use an SMTP relay service, which handles hosting the mailserver for you. This isn’t really a viable option though if you’re looking to self-host your mailserver, and learn about the intricacies of email.

For users needing to get a mailserver up and running simply and fast, free DNS hosting services like HexName allow registration of subdomains (like example.hexname.com) and creation of necessary DNS entries such as MX records for mail handling, TXT for SPF/DKIM, SRV, and so on. Check out our blog on explaining how to set up a mailserver put simply.