You know the trick with gmail where the user can also use the address billg+whatevertag@gmail.com? Exim can do that too. It's called an email address suffix or sometimes plussed addressing. The official Exim manual shows it's pretty simple to add to your configuration. Their example shows just adding two lines to turn it on for local users:

Most sites hosted on a VPS don't actually want to deal with just the email addresses for local users (these are users that have a shell account on the system). It's more likely there's a configuration section using the redirect driver. This seems to be the relevant part of the configuration at PowerVPS in /etc/exim.conf:

If I change it to this:

Then I can use addresses like someone+foo@latenightpc.com. I got thinking about this because of some testing with the paypal sandbox, so I wanted to be able to do sandbox+seller1@latenightpc.com, etc.

Anyhow, adding those lines to exim.conf works but apparently it'll break when I do a cPanel upgrade. And I can't figure out how to do the equivalent change in WHM's Advanced Configuration Editor for Exim. Some days I hate WHM. The editor is horrendous. I've tried adding an equivalent section (since the editor doesn't let you edit existing sections) and I've tried messing with /etc/valiases/latenightpc.com but neither does the job. I'm no Exim expert (I've only messed with alias and .forward files mostly) but it should be a two-line copy and paste job that the WHM user interface has turned into hours of wasted time for me.

On the other hand, cPanel also provides the tool that let me figure out what to change. There's a useful mail routing troubleshooter in cPanel called "Email Address Trace" or "Email Delivery Route" (depending where you look). You can enter an email address and you'll get a picture that describes the routing of email sent to that address. This saves a lot of time sending test emails and staring at bounced mail headers. The names of the sections in the Exim config file show up in the email trace. In my case it said virtual_aliases_nostar was causing delivery to be blocked so that's why I tried adding the subaddressing lines there.

So if you're interested, you can have subaddressing too until your next cPanel upgrade just by editing the conf file and restarting the Exim mail server.