Google requiring all ‘G Suite legacy free edition’ users to start paying 5 years ago
I've used it for years, no issues with delivery to gmail or outlook.com / o365. They've got a bit about their delivery system mentioned here: https://mxroute.com/docs/#/Presales/ipreputation
It's pretty basic in terms of webmail/core functionality but it receives and delivers mail well from the various IMAP clients I use, which is all I need.