A question about a backup email server

Hello everyone,

I read the whole series about making a mail server on Ubuntu server and also read the tutorial on how to make a backup server (server B) in case the primary one fails. But my question is, if my primary one fails and I need time to make it up again, so the backup one, should be used to send and receive emails like the primary one, right? Or it is just SMTP? So I can just receive emails or could I also send emails?
But, if I can send emails too, the webmail is hosted on the primary server, so I can’t access Roundcube anymore, if it fails. So how could I send emails or even read my emails that I receive through the backup server? I don’t understand it.

Thanks in advance & Greeting