Well, unless it is specified somewhere that you should use their products, then I don't see why you should have to use it. If they come back after the fact saying that you must use their products, then you have a change in requirements that would need to be compensated for by that company.

Personally, I don't see what the problem is. A web mail provider is well known to be using FreeBSD, despite the IIS/MS reporting from Netcraft.