I have clients who require 100% uptime with email. I currently host on a VPS from PowerVPS. I've been through nightmares with companies such as Equivity. Power VPS has been great until yesterday. 6 hours of downtime during business hours, and today they are taking the server offline on purpose during business hours to fix a hard drive.
This cannot happen anymore. I'm afraid my clients have had enough of this crap and they will go host with the local phone company if I can't keep them up and running.
Who should I turn to? Is there a way I could have dual POP servers with 2 different providers so if one fails the other will kick in?
I use a 3rd party DNS (www.dnsmadeeasy.com) and have set up 2 mail records (MX) records with one a preference of 10 and the other 50. Then if server 1 goes down, the mail automatically goes to server 2.
I then either have the client enter in the IP of server #2 or I have mail.server.com point to server 1 and mail2.server.com point to server 2. Then the client can simply change their incoming mail server or set it up that their email program automatically checks both.
In my experience, it's really difficult to get near 100% uptime with e-mail, moreso than other services like websites or DNS.
It depends on how the server software you are using works...
With your conventional e-mail setup, the only machine that can receive your mail is the one that actually stores those messages. To have multiple machines writing or accessing that store, as two POP or IMAP servers would imply, you need to spread that store across multiple machines. There are a few (complicated) ways to spread this store across machine, including NFS or using a networked relational database.
So, basically, this is not cheap. DNS MX records add reliability that you won't lose mail, but not the reliability that you will be able to access it.