The license shall not restrict any party from selling or giving away the software as a component of an aggregate software distribution containing programs from several different sources. The license shall not require a royalty or other fee for such sale.

The Open Source Definition

Any provider that provides access via IMAP/SMTP works fine with a lot of email clients. I personally use mailbox.org.

As clients you can use Thunderbird, Geary or Evolution on the desktop and FairEmail or K9 (which will become Thunderbird for Android) on Android.

https://github.com/FossifyOrg