In that case, I can say mail-in-a-box is easy, however it does work kind of like an appliance and expects that the server is only being used for mail-in-a-box.

In that case, I can say mail-in-a-box is easy, however it does work kind of like an appliance and expects that the server is only being used for mail-in-a-box. You could move whatever's on your VPS now to a new one, and then put mail-in-a-box on the old one that's had the consistent IP? Alternatively, you could set up inbound forwarding to a personal e-mail (most registrars include free e-mail forwarding IME -- if yours doesn't, transfer it over to Namecheap), and then use an SMTP service for outbound mail from your domain (you could use Mailgun, SMTP.com, and many others). All of your inbound mail will be free, and if you find that you need to respond with your domain, you'll be paying way less than $6/mo for your outgoing.