new internet/email problem

I've had no problems with internet or email until the last month. Then communication between router and MacBook became a problem
First, reception seemed to be an issue, so I had to move closer to the router.
It seemed that the router was probably the cause, so I bought a new one. It didn't improve matters.
For a few hours after buying the new router, emails were OK but now I can only receive them. I can't send (except for ones which contain things like the word "test"). Only the smallest email gets through.
I've also noticed that internet connection speeds are really slow.
The problem can't be the router; I doubt that it's the ISP since my other computer, a Windows machine, is fine.
Any ideas?
The computer's an Intel MacBook running 10.4.11.
Thanks

Have you tried changing channels on the router? Do you live in an apartment or condo? Someone near may have just installed a router using the same channel.

Thanks for your reply. I have only one other network in range; the problem remains when I switch router channels.

Some more weird issues:
At a friend's house, my email and internet is fine which implies my MacBook isn't the problem. At home, though, my Windows computer has no problems with email and internet but the MacBook is affected as in my first post.