I just opened and responded to one of their e-mails today. While I was there I had some spare time so I clicked around a bit and ended up in the Preferences area. I don't recall what I clicked on next, but the result was the same as clicking on "Change e-mail" and there I was able to ask them to increase the number of e-mails they send to me. The highest level available was "7 or more per week," I believe.
I couldn't recall if they'd asked me my e-mail frequency preference before, so I decided to up it to "4 to 6 per week" and see what happens.
You have to re-enter your e-mail address (or, of course, change it if you wish) and indicate whether it's for home or work, but at the bottom of the small box is the frequency selection.
If I see an increase in mails, and thereby reward opportunities, I'll report back in!