Dear All

I am looking into migrating to Zimbra.

Current setup[mail received through courier-imap]:
Host: mailA [IP: 1.1.1.1]
mail handled by: mail.mailA
When mailA is down, its mail is handled by zimbraMailB [MX records setup]
Has accounts: a.b@mailA, b.b@mailA

New setup[Zimbra]:
Host: zimbraMailB [IP: 2.2.2.2]
mail handled by: zimbraMailB
Has an alias: a.b@mailA, b.b@mailA

Question:
How can I setup zimbraMailB to store messages destined to a.b@mailA, b.b@mailA in the aliases a.b@mailA, b.b@mailA when mailA is down?

How can I setup zimbraMailB so that once mailA is no longer in use to receive mailA's emails?

Your help is appreciated.

Regards