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Editorial Team
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Asked: 2026-05-16T08:07:49+00:00

Françoise Lefèvre@example.com I’m reading RFC 5321 to try to actually understand what constitutes a

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"Françoise Lefèvre"@example.com

I’m reading RFC 5321 to try to actually understand what constitutes a valid email address — and I’m probably making this a lot more difficult than it needs to be — but this has been bugging me.

               i.e., within a quoted string, any
               ASCII graphic or space is permitted
               without blackslash-quoting except
               double-quote and the backslash itself.

Does this mean that ASCII extended character sets are valid within quotes? Or does that imply standard ASCII table only?

EDIT – With the answers in mind, here’s a simple jQuery validator that could work in supplement to the the plugin’s built-in email validation to check the characters.

jQuery.validator.addMethod("ascii_email", function( value, element ) { 
    // In compliance with RFC 5321, this allows all standard printing ASCII characters in quoted text.
    // Unquoted text must be ASCII-US alphanumeric or one of the following: ! # $ % & ' * + - / = ? ^ _ ` { | } ~   
    // @ and . get a free pass, as this is meant to be used together with the email validator

    var result = this.optional(element) || 
        (
            /^[\u002a\u002b\u003d\u003f\u0040\u0020-\u0027\u002d-u002f\u0030-\u0039\u0041-\u005a\u005e-\u007e]+$/.test(value.replace(/(["])(?:\\\1|.)*?\1/, "")) &&     
            /^[\u0020-\u007e]+$/.test(value.match(/(["])(?:\\\1|.)*?\1/, ""))   
        );
    return result;
}, "Invalid characters");

The plugin’s built-in validation appears to be pretty good, except for catching invalid characters. Out of the test cases listed here it only disallows comments, folding whitespace and addresses lacking a TDL (ie: @localhost, @255.255.255.255) — all of which I can easily live without.

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1 Answer

  1. Editorial Team

    According to this MSDN page the extended ASCII characters aren’t valid, currently, but there is a proposed specification that would change this.

    http://msdn.microsoft.com/en-us/library/system.net.mail.mailaddress(VS.90).aspx

    The important part is here:

    Thomas Lee is correct in that a quoted
    local part is valid in an email
    address and certain mail addresses may
    be invalid if not in a quoted string.
    However, the characters that others of
    you have mentioned such as the umlaut
    and the agave are not in the ASCII
    character set, they are extended
    ASCII. In RFC 2822 (and subsequent
    RFC’s 5322 and 3696) the dtext
    specification (allowed in quoted local
    parts) only allows most ASCII values
    (RFC 2822, section 3.4.1) which
    includes values in ranges from 33-90
    and 94-126. RFC 5335 has been proposed
    that would allow non-ascii characters
    in the addr-spec, however it is still
    labeled as experimental and as such is
    not supported in MailAddress.

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