Disroot stopped using RainLoop long time ago when people became aware of a security bug in RainLoop and the fact that the RainLoop project appeared to be dormant. I think Disroot switched to SnappyMail, and then to Roundcube.
I wish I could find one that is just a little less ideological. Not that I have a problem with ideologies, but I have trust issues, and I've seen some people justify some shit in the name of their ideologies, and as such I don't fully trust someone with a policy like:
To be hosted on our servers you have to share our principles of anti-fascism, anti-racism, anti-sexism, anti-homophobia, anti-transphobia, and anti-militarism. Your projects must as well be based on the same non-commercial nature which keep our project alive, and on the desire to share and experience relationships and struggles, with all the patience it requires.
for instance not to monitor my email to ensure I adhere even when they pinky swear they don't. Again, not that any of that would apply to me nor that they do spy but just the thought that they may us enough. Before someone says some smartass shit, I'd say the same for the opposite but idk many nazi mail providers.
I guess I'll have to pay, idk, maybe forward email. I'd pay proton if they'd just let me use imap/pop3 without some stupid "bridge."
Long time ago Riseup, focused on activists, required two invite codes, probably to avoid abuse. They've relaxed it with asking for only one invite code. You should imho not be asking for invite codes on the Internet but ask your activist friends or read this : https://support.riseup.net/en/knowledgebase/1-accounts/docs/13-how-do-i-get-an-account