Good question. I have not given that much thought; I do not know what fraction of my e-mails are actually read. But the response rates I've experienced (one to three expressions of interest per fifty well-targeted, personalized e-mails sent) is sufficient.
I prefer to hone content with my intended human audience in mind, rather than tuning it for the filters. I feel the same way about so-called "search engine optimization."
The spam filters and the search engines are relatively clumsy right now, but as they continue to be "optimized" they will eventually perform the way users want. By writing content that users value receiving, you create a unity of interest between users and yourself that search engines and spam filters will eventually be smart enough to recognize. Might as well get in the habit now.
---J.
