Here’s something to look out for if you are building an international website where you are handling email addresses…
Email addresses, as you are probably aware, comprise of two parts. The “local part” (the bit before the @ sign) and the “domain part” (the bit after the @ sign). Therefore with the email address [email protected] , “richard” is the local part and “example.invalid” is the domain part.
The domain part of the email address can actually be 255 characters in length theoretically (it’s 63 characters for the actually domain name, then the TLD: however, you could have sub domains in force which can take it up to the 255 limit.
The local part of the email address can be 64 characters: bringing the count up to 319 characters. Allow an extra one for the “@” sign and 320… Just slightly too big to fit within the standard 255/256 character string field that has tended to be used. I’ve seen rumours that this section may be expanded to 128 characters, so plan for 383 characters.
Oh – don’t forget that Internationalised/Internationalized Domain Names are becoming popular in some countries and unless you want to handle conversion to/from Punycode or another storage format for the IDNs, then you’ll need to make your database store the email address in Unicode… However, that appears to cause a problem in some version of MySQL if you store it all in a Varchar field.
So in your programming language (such as PHP), you’re going to have to keep all the above in mind and then split the email address into two parts for storing in the database for sanity.
See http://www.faqs.org/rfcs/rfc2821.html, http://en.wikipedia.org/wiki/E-mail_address, http://www.santosj.name/general/stop-doing-email-validation-the-wrong-way/ and http://askville.amazon.com/maximum-length-allowed-email-address/AnswerViewer.do?requestId=1166932.