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Assuming this is an in-house server for in-house folks, then you're likely stuck with a solution like you have suggested.
However, if that is the case, then you likely don't need captcha. So, I'm guessing it's not the case. So, unless it has to be restricted as such or you are getting a free hosting account, I'd suggest another hosting service. You could also find out which free servers are being blocked and then add those to your registration preferences disallowing such email addresses for registering in the first place.
Maybe a better explanation of your particular situation might help us to better respond.