Mailmeteor

I bought this for the first time in November 2021. Since then we've bought another 5 instances for various emails we use.
It's a no-brainer for us: if you have small lists (sub-2K) or can split mailings over multiple days (2K per day) then you have the ultimate deliverability direct from your Google Workspace email.
If you're not running complex transactional emails but need well formatted emails with support for custom fields - basically mailmerge - then there's nothing we've found that comes close to this in simplicity and reliability.