OSX Yosemite periodically loses DNS info for my server
From time to time I have the same problem - specially with my own MX server.
More or less default DNS setup for this DNS entry.

Restart helps, but disconnecting and reconnecting the network cable works too.
You could also try to change your network location.

Every of these solutions suck, I know. But it's ATM the fastest one before figuring out the real problem (one of the thousands other problems that came with Yosemite).