User mail size

The user mail size is fixed to 64 char (on PHP code and on database mail field).

The format of E-mail addresses is formally defined in RFC 5322 (mostly section 3.4.1) and by RFC 5321. An e-mail address is a string of a subset of ASCII characters (see however the internationalized 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 email 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.

==> http://en.wikipedia.org/wiki/Email_address#Syntax

With Drupal 6, we can't store an email more longuer than 64 char (local-part + domain).