Re: Patch: Unicode email support (RFC 6531, 6532, 6533)

[Arnt Gulbrandsen]

I want to digress about one aspect here: SMTP/EAI and unicode 
normalization.

The general EAI approach to that is to avoid having the problem, ie. to 
define the SMTP/email extensions such that the problems become other 
people's problems.

Homoglyphs aren't an SMTP problem. Two codepoints may look the same, but an 
SMTP server doesn't have to think about which of the two domains is 
legitimate and which is the impostor. All that is the registry's headache.

De/composition are pushed to the DNS. The SMTP part just says: Convert to a 
IDNA a-labels in order to do the MX lookup, and otherwise don't mess with 
the bytes you received. (My patch uses ICU to convert to a-labels.)

That does leave a little trouble, mostly dealing with localparts, but also 
with local domains. Some of it will be tricky, e.g. doing unicode-based 
pcre on a system that doesn't use a unicode locale.

Arnt