I can certainly sympathize with your problem. It doesn't only happen with cable. Until a couple years ago I had serious reliability problems with a T1, but I digress. . .
I can see two additional options. One, if your hosting provider is willing to add your Zimbra box's IP address to an allowed list for relay, then you can have your outgoing mail still go thru that host. Depending on how friendly and cooperative they are that is a possibility.
The other would be to consider hosting your email with one of the zimbra hosting providers that are out there--that way you'd actually have all the zimbra functionality in a place that's outside the unreliable connection. Of course, when your internet connection goes down you would have nothing again. . .
None of these is perfect, but it's all my Friday-tired brain can come up with!
