Iphone receives email but laptop doesn't

I have three email accounts: 2 on yahoo and one gmail. I do 90% of my email sending and receiving via entourage on my MBP. Several times per week, I'll find day-old emails on my iphone that somehow never made it to entourage. My hunch is the iphone is grabbing them and taking them off the server, so when I go to check my email in entourage they are no longer there. Is this a reasonble assumption and if so what can I do to correct?