Thank you for the update! We will consider this functionality in upcoming releases if it will be popular.
Everyone, please continue voting for this feature if you consider it important.
The feature request in our backlog for the next release.
Since Plesk Onyx you will be able to install memcache as a Docker container and configure http(s) proxy rule for it to map to your web app.
Docker and proxy rules are available in Plesk Onyx Preview already, you can try it here https://talk.plesk.com/threads/plesk-onyx-preview-and-feedback.337172/
Could you clarify what is the main value for you in Rainloop comparing with RoundCube?
SEDamian Hinz commented
Branding in Rainloop is quite easy.