I always just write only the domain in the subject. Usually I write new messages for every domain. Some types of domains deserve a different sales approach. It's usually about 3 paragraphs or 800 words, sometimes way shorter too. Try to make it fun to read even if they know everything already. If it's a really valuable name and I'm talking to bigger companies I'll keep it very short, a sentence or 2. Typically not too many numbers or facts. Just basic info about domains and how they might want to just re-brand all together. Usually always link to namebio. (no specifics) so they can look at prices, or similar active flippa auctions. Also general tips about advertising, getting more traffic, really anything to help them out beyond just domain name. If I do outreach I always put a price on it. Typically, if their site is really pathetic I'll include "Asking 1,250 (always make it really exact, 1,250 rather than 1,200 [psychology]) I'll include some extra tips, pointers, ideas etc at that price, otherwise I'm open to negotiations."