Never hit the 300 limit, I had one small client with a multitech FaxFinder and the clients using gmail, the faxfinder (like IP office) doesn't let you send SMTP email directly via gmail so I used GMX, but the service was slow, with their one page faxes showing up hours later than the when it was faxed. Using Hmailserver on a local dedicated PC fixed the issue.
Btw, not sure what you guys do on small customers for remote support, but on our really small ip office customers that are not using voicemail pro, I always put in a "remote PC" (that's what i call it anyways), it's basically a refurbished old windows xp pro OS (small form factor PC) loaded up with Manager and logmein.com, we set up that PC to automatically boot after any power loss and it's $99. Cheap thing to add to make my life easier to support my customers. On really, really, small customers we just statically assign the IP500v2 to their network and can always do a join.me session for support via the web manager, we do this mostly for muppet mode customers since that at least has the built in web manager.