I just went through the same issue; incoming mail - no problem, outgoing mail - didn't. I had to call my ISP to check: STMP server address, port, and whether or not they use SSL. Once I corrected these settings - problem fixed!
I have a converted Sky Netgear Router GT it receives in coming without a hitch, but will not send out, god no's why. so I switched back to me netgear V2 it sends out without hesitation.