Hmmm.... I had something like this a couple of weeks ago. I could receive mail but not send it - all the systems (Linux and M$) were affected so I knew it wasn't OS specific. Thought it must have been the ADSL router so upgraded the Netgear firmware - still no joy. Tried on dialup - still blocked.
Spend a dog's age on the blower with my ADSL/ISP's collection of half-trained morons
"You should reinstall Windows"
"I'm not running Windows"
"TCP/IP isn't supported on other operating systems"(sic.)
Eventually found someone who knew what they were talking about and he suggested I switch to another of the smtp servers - problem solved. The original smtp server for some reason has stopped authenticating me.
When something suddenly stops working (esp. in Windows)
1) DON'T immediately start fiddling!
2) Reboot - if that doesn't work..
3) Cold boot - if that doesn't work
4) Shut down and try again in an hour - if that doesn't work
5) Try again next day.
PCs/OSes are now so complex that they are almost organic - they have moods and end-of-month blues and sometimes hiccough for all sorts of obscure and transient reasons that often go away by themselves eventually. Start tinkering only if the glitch is very persistent - most scrambled systems that I've seen have been due to user fiddling when patience would have been better.
PS: If you system is unstable or unpredictable try running Memtest86
http://www.memtest86.com/ - Memtest should run without ANY errors - if you DO get errors you may need to tinker with the FSB speed or memory timings or even replace cheapo no-name memory with branded (and alas more expensive) memory like Geil or Corsair.