Too Lazy Didn't Name

Mailbox.org > proton. Fite me

Works great for me. I had it running in a snap for awhile, but now I just have it in a proxmox Debian container running a LAMP stack. I have over a terabyte of stuff saved and multiple computers syncing too, so its well used.

Ye

In the US at least, ISPs can force you to use a specific router and software restrict certain functions.

I have AT&T and they do this to me, I just have my own router behind theirs. Might be what OP needs to do.