Also, I use its under-publicized 10GB of free space (i.e., additional to the 10GB of mail space allowance) to more than comfortably host LDAP data such as my Joplin data, and Floccus bookmarks.
The other features (files, file sharing, calendar) are also well designed and get out of your way.
After that it’s much easier to move provider again.
Google would probably justify this as security, and not necessarily unreasonably, but it has a clear anti-competitive effect too. The security concerns would be more credible if they made it easy to debug this, like giving a useful error message back to the sender stating what the missing security criteria are and having a clear process for appeals (like if you got unlucky with an IP address, or if you are missing a specific security measure on your domain).
I have my domain pointed at Apple Mail, though. That probably helps.
My parents have a domain bought from OVH (they had to move away from their ancient ISP email address) and just use the free email service that goes with it. OVH is big enough that its email servers don't get blocked by the other main providers (it's a different matter if you host your own mail server on OVH servers) and they have not had any delivery problem.
For myself, I run my own mail server and have not had delivery problems for years now (even changing servers, so it's not just a case of IP reputation improving over time).
We do sometimes have delivery problems at work (also running our own mail servers, hosted at Scaleway) but it's to be expected at the kind of volume we have, it stays within quite acceptable levels.
They are to me the most legit alternative to Google Workspace/Office 365, run on their own infra and they've been here forever.
Dead simple email that just works. Their webUI is fine, but I almost exclusively use it on iOS or macOS with the default mail app. They also have some other features (calendar, office suite, video calls) that I don't use. I really like the option to create up to 25 email aliases.
So far, it has worked consistently with no problems. The only annoyance is iyt doesn't seem that you can break multiple icloud-hosted mailboxes out into their own GUI mailboxes in the Mail client - they all get dumped into a Mailbox called 'Cloud'
Congrats, you have more choices now :P
I have positive experience with both posteo and mailbox.org.