Last month a bill in the Israeli Knesset would have required ISPs to provide portable e-mail addresses, analogous to portable phone numbers that one can take from one phone company to the other. As I noted at the time (also cross-posted and discussed here on CircleID), e-mail works differently from telephone calls, and portability would be difficult, expensive, and unreliable. So I was wondering, idly, if we really wanted to provide portable e-mail addresses, how hard would it be?

Telephone number portability works by looking up the number in a global shared database before routing each call. The Internet has a global shared database in the DNS, and each time a mail message is sent, the sending system looks up each recipient address in the DNS to see how to route it. But the lookups are just for the domain name, the part after the @ sign, not for the whole address.

If we could adjust the syntax of mail addresses so both the user part and the domain got a DNS lookup, then it would be technically straightforward to port an address from one provider to another. I realized that some mail systems have done this all along. If someone’s mail address were [email protected] we can put the user name into the domain, and make the address [email protected], where the something is arbitrary. I’ve offered this option to my users for years, as a way to do tagged addresses, with the new form of the address equivalent to [email protected] at delivery time, something that turns out to take one line of code to implement in the mail system I use.

Several ISPs offer user mail addresses in this form. Panix in New York and Demon Internet in London have done so since the early 1990s, and there are probably others I haven’t run into. All of them do this to provide multiple addresses for their users, but with some degree (well, a large degree) of adminsitrative effort, they could arrange their DNS so that specific users’ mail is delivered to other mail systems.

If you know in advance that you want to keep your mail address when you switch ISPs, you can register your own domain for $10 - $15/yr, but for people who aren’t that adventurous, mail addresses with the user name after the @ sign offer, in principle, portable addresses too.