That is very similar to my letter, which I have had some nice success with. While it could possibly be improved, I doubt it is what is holding you back. One thing, though, if you are using one of your domains for for your email and/or in your sales letter signature, make sure it resolves. On days I send out emails, I find that I get more traffic to my domain that I use for email and in my signature. Other than that, your sales email makes sense and seems like it came from a real person.
Would you care to share the domains you are pitching and how you go about finding potential buyers? These two factors are what have made the biggest difference for me.
I find my success begins with the domain. There are a lot of ways to mess up even if you have a good domain, but pitching domains that are at the very least average is required for success. I send out 20-30 emails per domain, and I nearly always get multiple responses for a .com that makes sense. .Org's and .net's that make sense and that would truly be an upgrade for the potential buyers also do well. Anything else is a hard sell, for me at least.
The other day, someone sent me an email re: sms-free.ws and free-sms.mx as a domain "I may be interested in." While I do have a domain that contains "sms," I would never be interested in those two, and IMO, people spamming stuff like that is hurting the rest of us...
. If that held true, though, and if I had paid reg fee for those 10, that could still be a decent ROI, depending on how long it took me to find prospects. 
