Another happy user of Purelymail here. The GUI is unpolished, but deliverability and reliability have been excellent for me in the few years that I've used it. Unlimited custom domains and inexpensive.
It's not suitable for bulk sending, but for transactional and personal email, I've found it meets my needs.
Be warned, however, that it appears to be a one-man band, and if he goes under a bus...
I'm also a happy user of Migadu. More polished GUI and more features. I've never hit their limits, so not sure how suitable it would be for a medium-sized enterprise. As with Purelymail, it's not intended to be used as a bulk sending service.
Did I read somewhere that Scott is training his brother as a failsafe. Joking aside, I love the service as it is just what I need for not mission-critical stuff, as I tend to board domains and love to experiment with stuff. Would love somewhere else instead of Roundcube, but not sure is there anything modern enough. The only killer feature I need is super fast power search.
[1] https://purelymail.com