I would rate the tool's stability a ten out of ten. 

The stability is good. We have their service, which is on Amazon Web Services. We also have some Red Hat Linux appliances that we use in conjunction with that, on-prem, so it's kind of like a hybrid. 

It's a very stable solution and you can scale it as much as you need. It has features that can take over the DKIM and SPF records, which means that you don't have a scaling limit of 10 on SPF entries.

They have a macro approach and it works very well.

This is a mature product and it is stable.