I've not heard of mail in a box but I'm using Mailcow myself. They have a full docker compose and update script that will tear down and upgrade the stack itself. Been using it for years with little issue.
It's a good thing you have a mail forwarded already as you risk getting immediately grey listed without one for outgoing mail. I'm using SMTP2Go myself and it works great. They have a free tier that works for very little volume outbound. I may need to see about moving to ImprovMX if it makes sense to do so.
My use case is basically for internal system alerts and testing for work mainly. I don't want to rely too heavily on my internal mail server because if it blows up for some reason, I don't want to have important information in it that could be costly (time consuming) to recover.