Gmail-style aliasing with postfix

One of the less known features of Gmail is the ability to receive e-mails sent to particular aliases of the main e-mail address, e.g. mails sent to john.doe+22@gmail.com will get to the main mailbox, john.doe@gmail.com. (NB: the address is made up, hopefully this is not someone’s real address).

How would you replicate such feature on a local node you manage? I won’t cover all the details, at most I will just scratch the surface a bit. When using Postfix (the default MTA installed with Redhat/Fedora/CentOS), one must first look to the place where such functionality can be put in, the virtual_alias_maps option in the configuration file (main.cf).

The mapping concept in Postfix is common throughout its options. A “map” must be able to provide a translation between an input (e.g. an e-mail address) and some desired output (e.g. the mailbox location on the disk, the real destination e-mail). The “virtual” part is understood by Postfix as anything that is not tied to a real Unix account. So, in order to be able to provide the Gmail-style aliasing, one must essentialy create a (virtual) conversion map that would relate an input (addresses with “+“) to the desired output (the main mailbox).

Looking back to the wildcard e-mail addresses that we may get e-mails sent to, the only solution to get them matched to the real destination is by using some sort of regular expression. Simple map types accepted by Postfix (e.g. hash tables) perform exact matching, e.g. we may be able to alias john.doe+1 to john.doe but not john.doe+2, neither +99 (well, we are able, but we must put in a line for every such alias). Postfix does support regular expression matching through a built-in module, so we may have in the configuration something like:

# cat /etc/postfix/main.cf | grep virtual_alias_maps
  
virtual_alias_maps = regexp:/etc/postfix/virtual_alias

And the “virtual_alias” file may contain something like:

# cat /etc/postfix/virtual_alias

/^sample.name+(.*)@brainware.ro$/	sample.name@brainware.ro

This solution just works, but there are some drawbacks to it:

  • One must manually edit the “virtual_alias_maps” file to add new aliases. This is not practical for large installations with e-mail accounts created and deleted from web interfaces.

  • The regular expression matching is done within Postfix, theoretically slowing down the mail system throughput. On a busy server with thousands of such aliases this may become a noticeable issue (maybe not much of a serious problem with modern hardware, though).

For both problems, the solution is to offload the address matching to a database installation (e.g. MySQL, PostgreSQL) and do the regex matching from a stored procedure. I may come back to this on a later date, though. But for now, thank you for your time!

Leave a Reply

Your email address will not be published. Required fields are marked *