I have the same problem that many folks are having with the factory droid e-mail client going to sleep after a few hours and not being able to receive e-mail until they reboot the phone. I've searched the web and found that many are recommending K-9 e-mail client or forwarding all their addresses through google. I've been using k-9 the past two days and it seems to work very well. I noticed as long as i have K-9 open with the push service enabled and its constantly checking for e-mails...my factory e-mail client works too. So long story short, now that i've installed k9 and keep it running, the factory mail client is functioning perfectly.
I still prefer the factory mail client over k-9, is there a possibility that there is a service or something that k-9 is running which is allowing the factory mail client to function perfectly?