Backup line for Brightmail VM - Best practice?

Hi All

I am not quite the IT professional and need some advice on what the best way of doing this is.

We have had issues with our fibre connectivity and to increase uptime we have addded another fibre line from a different ISP. Obviously this means we have two public IP's (for mail) both going in to our firewall.

With this said, am I correct in saying I would need to create two MX DNS entries (e.g. mail1.abc.com and mail2.abc.com) with mail1 having a higher priority, lets say 10, and mail2 a priority of 20? I will then also have the ISP's add the PTR records.

On my brightmail device, what should I make the host name? mail1 or mail2? Or something else?

Hope what I am trying to do makes sense above. 

Thank you.

Regards,