Hi, David

I took a quick look and here's the issue. (It's a pretty common one, to be honest.)

If you go to Settings > Departments, and look at the Options for the department you have set up, the SMTP Account is set to "Use Brand's SMTP Account". However, you don't have an SMTP account set up for your Brand. If you change that to one of the other SMTP accounts you created, replies will start sending out. 

You can verify this by getting a reply, of course, but also going to Settings > Diagnostics and checking the Log Files tab. You should start seeing SMTP logs being generated.