Hi Erik!
I imagine you're going to be flooded with advice to grab a lawyer right quick, and that it's just smarter to change your domain name immediately. I'll offer a dissenting opinion, after having dealt with one too many wannabe copyright troll lawyers myself. Here's my response to the last barrister who tried something similar with a domain I own:
> Robert,
>
> We believe the statement on our page specifically disclaiming any
> authorization by or affiliation with http://your.site is more than enough
> to prevent any possible confusion between http://your.site and our product.
>
> From Graham Webb Intl v Emporium Drug Mart, 916 F.Supp. 909 (E.D. Ark
> 1995):
>
> "The Court likewise rejects Graham Webb's argument that the warnings Drug
> Emporium has posted disclaiming any affiliation with or authorization by
> Graham Webb are ineffective and not a question properly resolved by
> summary judgment. While it is true that the effectiveness of a
> disclaimer in a Lanham Act case may generally be a question of fact, see
> Home Box Office, Inc. v. Showtime/Movie Channel, Inc., 832 F.2d 1311,
> 1315 (2nd Cir.1987), a disclaimer expressly declaring that the seller is
> "not affiliated" with the owner of the trademark or is "not an
> authorized distributor" of the trademark owner's products has been held
> to be an effective means of preventing confusion in the minds of
> consumers as to affiliation with the owner of the trademark. Matrix
> Essentials, 756 F.Supp. at 282. As in Matrix Essentials, the only
> reasonable conclusion in this case is that Drug Emporium's warnings to
> consumers disclaiming any affiliation with or authorization by Graham
> Webb effectively prevents likelihood of confusion in the minds of
> consumers as to sponsorship or affiliation."
> ".
>
> Please view the amended site and let us know if you disagree.
Long story short: read up on the Lanham Act and all it's particulars, talk to a lawyer if you have one on retainer (a trial attorney is expensive, but legal aid and advice is relatively speaking much cheaper), and most importantly, don't freak out - this guy's claim is so vague that he doesn't have a chance of shutting you down (note: IANAL and also not your lawyer, this is just my opinion - but I've had two separate side projects shaken down by wannabe copyright trolls, and I can at least confirm that my approach has worked for me in the past and stopped any potential legal action in it's tracks.)
Somewhere on your main page, perhaps in the footer, specify that you have absolutely no affiliation with his product. Notify him that you've updated your site to clear up any potential confusion between your product and his, and ask him if he can provide any proof of customers for Caseweb confusing Caserails for his product, so that you and your counsel can further evaluate his claims in hopes of reaching an amicable resolution. His response will be illuminating, I'm sure.
Good luck.