No, an email domain cannot officially have an underscore. The global rules governing domain names, which form the latter part of an email address (after the '@' symbol), strictly prohibit the use of underscores.
Understanding Email Domain Naming Rules
An email address is typically structured into two primary components: the local-part (the username or identifier before the '@') and the domain (the registered name that identifies the mail server after the '@'). While the rules for the local-part can vary depending on the specific mail server, the regulations for domain names are much more stringent and are universally enforced by the Domain Name System (DNS).
Why Underscores Are Not Permitted in Domains
Domain names must adhere to the technical specifications defined in various Internet Engineering Task Force (IETF) Request for Comments (RFCs), particularly those concerning the Domain Name System (DNS), such as RFC 1035. These foundational standards dictate the permissible characters for hostnames and domain labels.
- DNS Naming Conventions: According to DNS standards, a domain name can only contain a limited set of characters:
- Letters (a-z)
- Numbers (0-9)
- Hyphens (-)
Domain labels must begin and end with a letter or number, and hyphens are not allowed at the beginning or end of a label. The underscore character (_) is explicitly excluded from this permitted set.
- Server Compatibility: This restriction is fundamental because domain names are resolved by the Domain Name System (DNS) and hosted on servers, where the underscore character is specifically not permitted for domain naming according to established internet standards. Attempting to use an underscore in a domain name would prevent it from being properly resolved by DNS, making email delivery to that domain impossible.
Distinguishing Domain from Local-Part
It's important to differentiate between the character restrictions for the domain part and the local-part of an email address.
Local-Part Character Rules
In contrast to the domain, the local-part (the section before the '@' symbol) has more lenient character rules. Many email service providers and mail servers do allow underscores, along with other special characters, in the local-part of an email address.
- Examples of email addresses where the local-part includes an underscore (and is typically valid):
However, even for the local-part, it is generally considered a best practice to stick to alphanumeric characters, hyphens, and periods to ensure maximum compatibility across all email systems and avoid any potential parsing or delivery issues.
Summary of Email Address Character Rules
To provide a clear distinction, here's a summary of character rules for different parts of an email address:
| Email Address Part | Permitted Characters | Underscores Allowed? |
|---|---|---|
| Domain | Letters (a-z), Numbers (0-9), Hyphens (-) | No |
| Local-Part | Letters (a-z), Numbers (0-9), Hyphens (-), Periods (.), and often other special characters such as _, +, !, etc. (server-dependent) |
Yes (often, but not universally guaranteed) |
Best Practices for Email Addresses
When creating or managing email addresses, adhering to widely accepted standards ensures universal deliverability and avoids technical complications.
- For Domain Names: Always use only letters, numbers, and hyphens. Strictly avoid underscores, spaces, or any other special characters.
- For Local-Parts: While many servers permit underscores, sticking to alphanumeric characters and periods (e.g.,
[email protected]or[email protected]) generally offers the broadest compatibility and minimizes the chances of errors or deliverability issues.
