Support matrix

The tables on this page are built from the packages as the site builds: the syntax rows from validator-syntax's test corpus, the provider rows from the classifier's registry, and the rest by running the packages on fixed examples. They describe the versions these docs are built against, and they change only when the packages do.

Address syntaxCopy Anchor Link

One row per address feature of RFC 5321 and RFC 5322, plus the forms a preset accepts outside them. A cell says whether the preset, with its default options, accepts the corpus fixtures that are well-formed examples of the feature: Yes for all of them, No for none, and Partial, with a count, for some. The example is one of those fixtures, with any control characters shown as their Unicode symbols, like ␀ for NUL.

supportMatrix() returns the same rows, with every fixture. Presets describes each preset's rules, and options such as allowComments and allowIpLiteral change what a preset accepts; previewSyntaxOptions shows how (see Fixtures).

International addressesCopy Anchor Link

RFC 6531 extends SMTP to UTF-8: non-ASCII characters in the local part, and domain labels written as U-labels (§3.3). Each cell runs isValidSyntax on the example with the preset's defaults, then with the option that covers it.

See International addresses for what each option accepts and the length caps that still apply.

Null MXCopy Anchor Link

A Null MX (RFC 7505) is an MX record whose host is ., published to say a domain receives no mail. validator-dns checks it, not validator-syntax, so presets don't apply. These rows run checkDns on a stub resolver that answers with the MX records shown and one A record.

See What's checked for the order validator-dns decides in.

ProvidersCopy Anchor Link

The classifier's provider registry: each entry's domains and the local-part rules the sanitizer's provider-aware normalization uses. Provider registry says what each field means.

SourcesCopy Anchor Link

Each entry cites its domains, its MX patterns, and every rule that differs from the default, from providerSources. "DNS lookup" marks a fact the provider documents nowhere, read from its DNS instead. Checked is when the fact was last compared with its source.

Not supportedCopy Anchor Link

Mailbox probingCopy Anchor Link

No package asks a mail server whether a mailbox exists by sending SMTP RCPT TO. validator-dns's SMTP probe stops after EHLO: it learns whether a mail server answers, never whether a mailbox exists.

RCPT TO doesn't answer that question reliably:

• Many servers accept every recipient and bounce later, or deliver anything for the domain to one mailbox, so a 250 doesn't mean the mailbox exists.
• Greylisting and rate limits answer with temporary failures, so a 4xx doesn't mean it doesn't.
• Providers treat repeated probes from one address as abuse and block it, which then stops that address's real mail too.

The only proof that a mailbox exists, and belongs to the person who typed it, is mail they receive: send a confirmation link.