If you want to do transactional emails, I love Mailgun. If you use this link, you'll get 30k free emails a month vs 10k you normally get: https://www.mailgun.com/google
If you want more marketing emails, for free starting up I've heard great things about Mailchimp and MailerLite
Beyond the free tiers, when wanting to get more advanced in emails, I've heard great things from many bloggers about Convert Kit https://www.clackapedia.com/convertkit
. It's expensive but easiest to use out of all the automation platforms.
If you want Email Automation on the cheap and are very technically minded, you can try Mautic www.mautic.org
open source self hosted marketing automation platform. I'm playing with it right now and While it has a BIG learning curve, it's easily the most powerful one out of the bunch.
If you want to use Mautic, I recommend taking this course on Udemy. https://www.udemy.com/the-ultimate-guide-to-mastering-mautic
IMO It's worth the ~15-25 bucks to break down how to use Mautic quickly, it's also less than you'd pay for a month of ConvertKit.