Separate email server from web server

Hello,
I'm thinking about the opportunity to move all the email accounts currently on "server A" on a different server "server B".
"server B" is a server with imap and smtp services and without webmail client, while "server A" is a web server with email services disabled.
What I wanna do basically is to make migration transaprent to my customers, so I'd like to reconfigure roundcube on "server A" to fetch email from "server B".
How can I tell roundcube to fetch email from server B instead of server A?
If I disable email services on server A does the webmail client interface remain available to my customers?

Btw password sync is not a concern.

Thanks