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as title says, is it free, or do I need to pay something for it?
I afraid not
"Everyone you meet is fighting a battle you know nothing about. Be kind. Always."
Are there any other panels to easily install email/rainloop?
iRedMail, Mail in a Box or Mailcow
ik moet poepen
Try Cloudron 2-weeks for free, no credit card required. So It's a 2-weeks free trail
You could try Yunohost. there's no containerization of applications/services/resources, but Rainloop is one of the apps you can deploy with it:
For better security than Cloudron, you could try Sandstorm (it's free), but it doesn't have Rainloop. It has a bunch of other apps:
I've researched Sandstorm / Yunohost / Cloudron and the ilk, which is how I know about them, but haven't used or tried any yet. I've settled on Sandstorm though, and will be deploying it soon...
Also, Bitnami has installers for all kinds of applications that you can install on any linux dist, no Rainloop, but there is Roundcube:
Do you need an email server as well?