How to use it
- Paste your addresses into E-mail addresses, one per line.
- Read Valid syntax and Syntax errors for the counts.
- For each address marked invalid under Verdict, read the Reason and correct the address.
What the rules check
An internet mail address has a local part, an @ sign and a domain (RFC 5322, section 3.4.1). The local part is a dot-atom, which is letters and symbols with single dots between them, or a quoted string (sections 3.2.3 and 3.2.4). The domain is a domain name or an address literal in brackets, which holds an IPv4 or an IPv6 address (RFC 5321, section 4.1.3).
- A mailbox may not need non-ASCII characters in SMTP, so a non-ASCII letter fails here (RFC 5321, section 4.1.2). Internationalised addresses need extensions this page does not check.
- Domain labels hold letters, digits and hyphens. A label may not start or end with a hyphen, and the underscore is not permitted (RFC 5321, section 4.1.2).
Length limits
The local part is at most 64 octets and the domain at most 255 octets (RFC 5321, sections 4.5.3.1.1 and 4.5.3.1.2). A domain label is at most 63 octets (RFC 1035, section 2.3.4). The whole address is at most 254 octets, because an SMTP path of 256 octets includes its two angle brackets (RFC 5321, section 4.5.3.1.3).
Syntax is not deliverability
An address in the right form may not exist, and a mailbox may refuse mail. Only the mail server of the domain can tell, and this page never connects to a server. The page also takes a bare address: a name written before the address in angle brackets is not checked as one.
Example: three addresses checked
[email protected] has a dot-atom local part and a domain name, so its syntax is valid. [email protected] has two dots in a row in its local part, which the dot-atom form does not allow (RFC 5322, section 3.2.3).
user@my_host.example fails because RFC 5321 does not allow an underscore in a domain label.
| Address | Verdict |
|---|---|
| [email protected] | valid |
| [email protected] | invalid |
| user@my_host.example | invalid |
Frequently asked questions
Does a valid address receive mail?
Not necessarily. Syntax only shows that the address is written correctly. Whether the mailbox exists is known only to the mail server of the domain, and this page does not contact it.
Why is [email protected] invalid?
The local part is a dot-atom: one or more characters from a set that includes letters, digits and symbols, with single dots between them. Two dots in a row, or a dot at either end, break that form (RFC 5322, section 3.2.3).
Can an address contain a space or a comma?
Only inside a quoted local part, such as "john doe"@example.com (RFC 5322, section 3.2.4). An unquoted space or comma is not allowed in the local part.
Why does a domain with an underscore fail?
SMTP domain labels are letters, digits and hyphens, and RFC 5321 (section 4.1.2) says that the underscore is not permitted in them. Some DNS names use it for other purposes, but a mail domain must not.
What is the longest address allowed?
254 octets for the address itself. The local part may be 64 octets, the domain 255, and an SMTP path of 256 octets holds the address with its two angle brackets (RFC 5321, sections 4.5.3.1.1 to 4.5.3.1.3).
Does this checker send my addresses anywhere?
No. The check runs in your browser, and nothing you paste is sent or stored.
Sources
- RFC 5322: Internet Message Format
- RFC 5321: Simple Mail Transfer Protocol
- RFC 1035: Domain Names, implementation and specification
Figures and text last checked on Tuesday, November 10, 2026.