Email Subaddressing

Warning: This blogpost has been posted over two years ago. That is a long time in development-world! The story here may not be relevant, complete or secure. Code might not be complete or obsoleted, and even my current vision might have (completely) changed on the subject. So please do read further, but use it with caution.
Posted on 04 Feb 2014
Tagged with: [ email ]  [ exim ]  [ mail ]  [ subaddressing

Sometimes you are looking so hard for a solution, that you won’t even see them even if they punched you in the face. Email Subaddressing is one of those issues I couldn’t get fixed.

Subaddressing, which is something that for instance gmail uses, makes that you can use a plus-sign + to add additional data to the local part of an email address. Mail that is sent to will be delivered to But it allows me to see that this email was sent to me+yourwebsite, which means that this email-address was the one i’ve used to register myself on onto your website for instance. This makes it very easy to either filter out emails based on the to-address, track where email-addresses came from (or where the spam was originated from, or who sold your email address etc).

The thing is: most registration forms on websites  are complete and utter crap and do not allow a + sign (or worse: where you must have at least 5 characters for the local part, or even some valid domains that are not recognized etc etc). In those cases, subaddressing do not work.

Most people solve this by using a catch-all on my domains aren’t an option too, because too many spammers sends email to random users.

So here’s the big punch I’ve received this week: you don’t HAVE to use a + sign for subaddressing. Might as well be another character (hell, even a letter!). I know I know.. obvious, but still it hit me just this week after seeing it in action by somebody else.

Since i’m using virtual domains and users, the only thing i needed to add to my exim.conf was:

local_part_suffix = +* : -* : _*

Which means I can use a plus, a hyphen or even underscores for sub-addressing.