If I was Zendesk, I would do exactly that, fight your trademark. Not that their objections will necessarily hold up. You could arguably defend it, but why bother?<p>If you say you are friendly with Zendesk, why did you name your product Zen-something if there is no strict affiliation between the companies, it doesn't make any sense, it should be lesson for others. The thing is that "being friendly" is not really tangible in legal terms. In fact it's the "being friendly" part that probably brought this on because if Zendesk hadn't heard of you and their customers weren't using Zenbilling then your application would probably have done through without hassle.<p>I know, and probably Zendesk do too, that what they are doing isn't strictly "nice" to you. But there wasn't any official partnership or other deal or agreement in place and that ultimately means there is no relationship between the companies.<p>What you are doing is right, find a new name and move on. But don't let it spoil your "friendship", and although probably there's no room for "favours" anymore, keep your mind open for mutually beneficial agreements instead.