Does capital/lowercase ever matter in emails or domain names?

Random

Right, as far as I'm concerned wWw.googLE.COM is the same as www.google.com or www.GOOGLe.com etc.

John.Burke@google.com is the same as john.burke@google.com is the same as JOHN.BURKE@GOOGLE.COM.

People are forever saying things like "Make sure the J in John and B in burke is uppercase or it won't work". To me this is complete nonsense.

Is there ever any case where lowercase/uppercase actually makes any different for domain names (not file names on websites) or email addresses?

Thanks

Gavin

Dns - http://www.dns.net/dnsrd/rfc/#rfc1101

2.3.3. Character Case

For all parts of the DNS that are part of the official protocol, all
comparisons between character strings (e.g., labels, domain names, etc.)
are done in a case-insensitive manner. At present, this rule is in
force throughout the domain system without exception. However, future
additions beyond current usage may need to use the full binary octet
capabilities in names, so attempts to store domain names in 7-bit ASCII
or use of special bytes to terminate labels, etc., should be avoided.

e-mail
http://tools.ietf.org/html/rfc5322

from Wikipedia

RFC specification

E-mail addresses are formally defined in RFC 5322 (mostly section 3.4.1) and to a lesser degree RFC 5321. An e-mail address is a string of a subset of ASCII characters (see however the internationalize addresses below) separated into 2 parts by an "@ (at sign), a local-part" and a domain, that is, local-part@domain.

The local-part of an e-mail address may be up to 64 characters long and the domain name may have a maximum of 255 characters. However, the maximum length of a forward or reverse path length of 256 characters restricts the entire e-mail address to be no more than 254 characters.[1] Some mail protocols, such as X.400, may require larger objects, however. The SMTP specification recommends that software implementations impose no limits for the lengths of such objects.

The local-part of the e-mail address may use any of these ASCII characters:

* Uppercase and lowercase English letters (a-z, A-Z)

So domain names - no.
local-part of email addresses - yes.

jhegarty

Left of @ can matter, but it depends on the email server.

Right of the @ it doesn't matter.

FruitLover

I don't think I've ever encountered a mail server that has been configured case-sensitive, and something like google mail definitely wouldn't be.