Email problem on local network

I have a home network using a DLink DI-664 revC router and one Desktop on ethernet link and a laptop on the wireless router. The problem appeared a few months ago with the laptop using the wireless connection. The laptop uses Windows XP, SP2 and IE7. I can receive emails on the laptop, but cannot send emails. I can use the internet to download files and order/buy items form internet websites. I have two separate email accounts, one for the desktop (myself) and one for the laptop (spouse). I can send an email to the desktop computer, from the laptop using my email address. I can send an end around email to the account on the laptop, from the laptop. Cannot send an email to any other email address. I have reset the router setup with Dlink support. I have discussed with my ISP, who claims the problem is not at their server location. Any suggestions, tools or Windows switches, IE switches or controls to try next?
Thanks,