You have a set amount of emails per hour, and a maximum per month (on the free plan). With a few dns settings it sends signed emails for your domains, DKIM and SPF.
The monthly limit is 12000 emails and I've never had a failed email since starting using it. Other than delayed emails the first day or two because it had to adjust the hourly allowance to prevent it being used for "spam".
If you want me to have a quick look though, just forward me that mailerdemon failure you're getting when sending from your personal and ill have a look. I have some experience with dealing with that sort of thing.