Valid user names

[EricG]

There don't seem to any threads for valid characters in user names. There is whole Wikipedia article on this.
RFC 6531 says local-part (before the @) may contain:

• Uppercase and lowercase English letters (a–z, A–Z), Digits 0 to 9
• These special characters: ! # $ % & ' * + - / = ? ^ _ ` { | } ~ (limited support)
• Character . provided that it is not the first or last character, and provided also that it does not appear two or more times consecutively.
• Special characters are allowed with restrictions [when quoted]: Space and "(),:;<>@[\]

Obviously no one allows much beyond letters and digits. The + and - are often used for sub-addressing. The . is often allowed, the _ sometimes. Scripts makes $ ? * ' difficult. Fastmail doesn't do dots. Most of the others look awkward, but ' is useful in Irish names.

Anyone have a list of valid extra characters for the common mail providers? What about Exchange?

[DrStrabismus]

Originally Posted by EricG

There don't seem to any threads for valid characters in user names. There is whole Wikipedia article on this.
RFC 6531 says local-part (before the @) may contain: ...

Usernames and localparts are not necessarily the same thing - a username needn't be anything more than what's used for logging in. At Fastmail usernames can't have dots, but localparts can.

[kijinbear]

Methinks the RFC is too permissive and needs to be more restricted in order to prevent common bugs and security issues. Seriously, two @'s in an email address would look ridiculous. So is any character that needs to be quoted/escaped. An email address should be easy to remember and type, not look like BBCode or (heaven forbid) a perl script!

RFC 5321:

a host that expects to receive mail SHOULD avoid defining mailboxes where the Local-part requires (or uses) the Quoted-string form.

I'm also ambivalent about "internationalized" (RFC 6530) addresses like δοκιμή@παράδειγμα.δοκιμή. Like it or not, the Latin alphabet is the de facto common denominator among all computer users on this planet. Email addresses like 甲斐@黒川.日本 only help to create insular communities and fragment the Internet, because the rest of the world can't even type that. Mail servers, of course, should deliver mail to any address that meets the relevant RFCs, but email hosting services have every right to refuse to give their users usernames and localparts containing uncommon characters.

So I'll have zero complaints as long as I get to use a-z, A-Z, 0-9, plus(+), minus(-), dot(.) and underscore(_). And I'm saying this as a person whose first language doesn't even use the Latin alphabet.