The left hand part of an email address, the bit before the
@ is case sensitive, in email in general. I've known that for a while - it seems to be an obscure-ish part of SMTP folklore.
Individual mail domains are perfectly at liberty to fold multiple distinct addresses into one, in their own domain, which is what most mail systems do:
bEnC all go to the same place
@hawaga.org.uk. This leads many people to think that the left hand side is case insensitive.
This is just as they are at liberty to do that folding in other ways: for example, gmail ignores
. in addresses, giving me
email@example.com. As well as
This came up on a mailing list (for browserid) that I watch, and I ended up being challenged in private email to cite a source. Luckily there's plenty of stuff around. RFC2821 section 2.4 seems to be the authority: The local-part of a mailbox MUST BE treated as case sensitive.
In the preparation of this blog post, I discovered something I didn't know before. It seems you cannot have multiple dots in a row in a plain email address. The RFC2821 production rules are:
Local-part = Dot-string / Quoted-string ; MAY be case-sensitive Dot-string = Atom *("." Atom) Atom = 1*atextwhere atext seems to come from the companion RFC2822 section 3.2.4. So
b...firstname.lastname@example.org not a valid address. Shame.