I personally used 7digital to rebuild my music collection. They sell good licensed mp3s.
I have absolutely nothing negative to say about them. The prices were decent, the files are boring DRM free MP3s, and they had a really good selection of music.
Honestly it looks almost exactly the same as when I used it for the first time like 15 years ago.
Since calendar is an app, but fundamental email service isn’t, one thing that I found is that apps can interact in ways that are completely unintuitive.
For example, I activated the ncdownloader app, and it caused mail to stop showing emails, or I activated nextcloud music and it stopped nextcloud news from updating.
You should check your logs, because usually when there’s a problem it will show up in there. The logs I’m referring to are in your administrator panel. It will be completely unintuitive as to what exactly is going on. The other thing that you can do is just pay attention to which apps you’ve installed, and if there are any that are a little bit unusual, just try to disabling them and seeing if calendar mail works after that.