In our case, we just use email. For emails, in the past I used Mailgun because they have a very generous free tier, but ultimately I switched to AWS SES.
We are a big fan of loops.so for sending transaction and newsletter emails, emails look great, has a great editor and lots of configuration logic.
Before that we used SES with semplates.io for transaction emails which was very cost affordable but not quite as flexible.
@pydubreucq yes but we don't use it for that reason. We searched for a good solution but we didn't found nothing better. Furher more the integration was very easy. So our decision was to use AWS SES