Cool idea - not new but convenient. Price is worth trying premium. Two concerns:

1) Custom email domains are susceptible to email spamming since you simply create a new address as you like. I would give up a little flexibility to ensure no one could simply create an alias email address on the fly. This would mean creating the alias first in the extension or interface, assigning a real email address and then use the alias on a site. This way spammers or bots couldn't just make a whole bunch of aliases connected to your domain. While I can appreciate the flexibility to create a new alias quickly this comes at a cost and is subject to abuse.

2) How can I manage my aliases using a mobile device? I haven't figured this out yet if it is indeed possible.