Email Marketing Journal has published a comprehensive guide to selecting email deliverability tools for your business.
Basically, these are tools that help you monitor and ensure that your messages are getting into your recipient’s primary inbox rather than their spam folder. They help you monitor your sender reputation, unexpected spikes in your soft and hard bounce rates, your engagement, and whether or not you’ve been blocklisted.
While the article doesn’t make specific recommendations regarding which services to use, it does an awesome job of quickly telling you what to look for and does offer some links.