I'm the editor of a statistical journal (currently it doesn't have an Impact Factor but will have one soon as far as our information goes). I have experienced that it is very hard to find reviewers for papers. Often people don't even respond to the invitation to review. Recently I invited a person I know very well to review, and they told me that the invitation ended up in the spam folder. This of course would explain why I often don't get a response.
I am aware that different spam folders work differently and I know that not all of my invitations for this journal end up in the spam. I am also Associate Editor for other journals (with OK Impact Factor) where my success rate getting responses is clearly better. So I'm interested in any tips regarding wording or subject lines of such invitation emails that lower the probability of ending up in spam folders.