Cannot send email through Wi-Fi. Can send email through 3G. iPhone

I am able to send email within seconds when Im connected through 3G. Back when I had the 1st gen iPhone, I was able to send on the Edge network. But with both phones, Ive never been able to send email when Im connected through Wi-Fi to any network (home, work, coffee shop). Receiving mail works as well as other Web functions. I just cannot send email, and in the rare case where it does work, its taken literally hours for a short email to send.
I used to think this was a weird phone thing and just ignored the problem since I always had the phones network to do the job. But just last week I got an iPad and the exact same thing is happening--cant send through Wi-Fi--and with my iPad, I dont have the option of 3G.
Why is it that its only sending mail that wont work and only with Wi-Fi?
iPhone 4, iPad 2 Mac OS X (10.5.8)