Hello.
My name is Andrey. I am experienced FreeBSD/Linux administrator. Have experience with different mail server installations based on Postfix as MTA, one of POP3/IMAP (Courier, Dovecot, Cyrus) and other components like clamav, postgrey, etc. I am not worked with amavis, but I am not think, that this will be a problem.
First of all, I need to ask you: What kind of backup email server you want?
- This can be backup MX server, which give an opportunity not to loose messages while primary is down. It not hold all messages and not provide POP3/IMAP services.
- This can be cold backup server, it not shown as email server in internet. Data periodically syncing from main server. When primary server fails, cold backup can be used after making some config and DNS changes.
Are you satisfied with how your primary server works?
As you noted your primary server is VM. In your own virtual infrastructure or at some provider?
Where you planning to host backup server?
I need to understand aim which you are want to reach by this backup server. This will let us to choose proper strategy of server build.
Thanks.