multiple / backup sql servers for sql server access

I am using this file:

dovecot-sql.conf.ext

and in there  I have a

connect = "host=<dbserver> dbname=<dbname> user=<dbuser> password=<dbpass>"

My <dbserver> was down and I lost mail access for a few days before I realized that there was an issue.  How can I specify a backup server so that if my primary sql server goes down, a backup sql server will be queried?

-- jack