An email subject only has one job, to get them to open the email. If you were selling a domain 'freediesel.co.uk', I'd probably put 'Diesel' in the subject.
Knowing that they have an interest in diesel, and the email might be from a potential customer for them, they'll read it.
You wouldn't use the word 'free' in the subject because that would increase the spam rating.
The email itself then has the job of selling not the domain, but the value of them owning the domain. What will it do for them?
Likely as not your email isn't going to arrive in the inbox of a person who knows much about domains, pay per click, seo, or perhaps even marketing in general.
For small companies you need the boss to read it. And you need him/her to see £ signs - be that from lead generation, branding, avoidance of competition, or something else.