Top Level Domain (TLD)

A Top Level Domain (TLD) is the last segment of a domain name, representing the highest level in the domain name system. It typically consists of two or more letters and indicates the purpose or geographical location of a website or entity. Examples include .com, .org, and .net. TLDs help categorize and organize websites on the internet, facilitating easy navigation and identification.

What types of TLD are there?

TLDs fall into two groups that are worth keeping apart, because they are run by different kinds of organisation under different rules:

• Generic TLDs (gTLDs) such as .com or .org. Some are open to anyone, others are sponsored and restricted to a defined community.
• Country code TLDs (ccTLDs) such as .de or .jp. These are derived from the two-letter country codes in ISO 3166-1 and are administered by a registry in or on behalf of that country or territory.

What are typical popular TLDs and their envisioned use case?

Generic TLDs and their envisioned use cases:

• .com - commercial/business websites, in practice used for anything
• .org - non-profit organizations, open registration
• .net - network infrastructure or service providers, open registration
• .info - informational websites, open registration
• .biz - business-related websites
• .gov - US government entities, restricted
• .edu - US-accredited educational institutions, restricted
• .mil - US military organizations, restricted
• .int - international treaty organizations, restricted

Country code TLDs:

• .uk - United Kingdom
• .ca - Canada
• .de - Germany
• .jp - Japan
• .fr - France
• .cn - China
• .co - Colombia. Widely marketed and used internationally as a short alternative to .com, but it remains Colombia’s ccTLD.
• .io - British Indian Ocean Territory. Popular with technology and startup companies, and treated as a generic TLD by most people who register one. Note that its long-term future is tied to whether the territory keeps its ISO 3166-1 entry following the 2024 agreement between the UK and Mauritius over the Chagos Archipelago. If you are choosing a domain for infrastructure that should still be there in ten years, that is worth factoring in.

Please note that this list is not exhaustive. There are well over a thousand TLDs, including many newer generic TLDs introduced since 2013.

Is there a list of mail relevant TLDs?

Spamhaus publishes reputation statistics that break down abuse by TLD, which is a good way to see which TLDs are currently dominated by spam and malicious registrations: Spamhaus Reputation Statistics.

This matters because some receiving systems and filtering rules score mail from badly-behaved TLDs more harshly, regardless of how well your own server is configured.

What is a good TLD for a mail server domain?

This is dependent on the use case you have. We recommend using customer domains for single mail systems of a company. If you want to run a general purpose system with hundreds or even thousands of clients, then .net or .com are a good choice, as they are long-established and carry no inherent penalty in spam filters.

Two practical points:

• Prefer a TLD with a stable registry and a low abuse rate. A cheap TLD with a bad reputation costs you deliverability you then have to win back.
• Keep the mail server hostname on a domain you actually control the DNS for, so you can set matching forward and reverse DNS records.

That said, the TLD is a minor factor. If a domain is abused for spam, the system will get into trouble regardless of which TLD it was set up on.