I’m able to reply just fine. They have a special address that goes to your inbox, and when you reply, it looks like you’re replying from that alias and not your main email address.

That was one of the first things I checked on, since replying is sometimes necessary.

You can also send by going into your Aliases and clicking “Send” by one of your aliases. This will open a dialogue that creates the recipient address you need to use in whatever email address is verified with Addy.

ETA: and to be abundantly clear, I’m using the free tier. This is all included in the free option.