IsTumblrDown hit with a Cease & Desist
IANAL, only an entrepreneur, but this should surprise no one even passingly familiar with trademark law.  Failure to enforce your trademark's registration can expose you to claims that your mark is no longer "in use."  Sure, this guy might be 100% innocent, but it won't matter when someone less innocent comes along and uses the fact Tumblr didn't enforce their trademark against this other guy to be annoying.It's easier to just shoot an email to your attorneys and have send a C&D than it is to enter into a more protracted, ad hoc conversation.  It costs virtually nothing to do this.  Some associate at the firm types it up using one of a bajillion templates, sends it out with a partner's name attached, and bills the client for You, too, can hire an attorney and pay them a few hundred dollars to reply!  Or you can try replying yourself.C&Ds are not legally binding, of course, and the recipient can choose to comply, respond, or ignore as they see fit.  Because of how trademark law works I wouldn't recommend "ignoring" since the complaining party is pretty much obligated to escalate matters.For example, a friend of mine created a parody website of a world-famous newspaper and predictably received a C&D.  This was more than just the trademark: it repurposed content, used the same typefaces, mimicked the same layout, etc.  They agreed to let him use it after he replied and agreed to include a prominent disclaimer up top stating that " trademarks used with permission of " and explaining that this was a parody.I don't know if the OP tried to do this, but I will say turning to the "court of public opinion" as a first course of action makes this outcome somewhere between incredibly unlikely to impossible.Tumblr isn't trying to be evil -- you're using their name, after all -- they're just trying to take care of it in the most time-efficient manner possible.  The #1 most time-efficient manner is sending a C&D.  For folks whose infringements are minor and inconsequential, it's likely that the #2 most time-efficient manner is to let them use the name in the domain with a prominent disclaimer and an explicit, albeit revokable, license.