I was wondering this myself. I belong to about 60 safelists. Some offer ~2-5 cred per email, others as high as 1000. I think that the lower the amount of creds offered, the better. If I can only mail 3000 members every three days, and I am earning 1000 credits for each email, I only need to read one email a day to be able to mail all the members I am allowed to. I think the chances of "my" email being the one that is read by fellow members is so low as to be non-existent.