Brevo - it does what it says, but should be more reliable

Civey is a market research company. We use Brevo for transactional mails to our voters and both a business and consumer (voter) newsletter. We have about 1 million voters in our panel.
The voter newsletter is business critical for us, because we get most of our voting traffic from it. The business newsletter is used for re-activation mostly. The transactional mails are an integral part of our user flow.