When you say, "From emails that I sent out", do you mean you personally sent all these 2465 emails out, or they are emails sent from your email account that have been returned?

If the latter then it would appear your computer is infected with a spam email virus.

What do you use as a firewall to prevent hackers, as an anti-virus to check emails and downloaded files for viruses, and as anti-spyware to check against spyware?

If you use nothing, or if your anti-virus and anti-spyware utilities are not kept up to date, you should review your protection methods.

If this is spam email being returned, then I would do this;

1] Turn off System Restore to delete all your System Restore backup files where viruses and spyware can hide. To turn off System Restore right click your "My Computer" icon, select Properties, then click the System Restore tab and select the "Disable System Restore" option. Click Apply then OK.

2] Head over to Housecall at http://housecall.trendmicro.com and perform a complete online anti-virus scan. Let the scan delete everything it finds.

3] Head over to Ewido for an online anti-spyware scan at http://www.ewido.net/en/ . Delete everything the scan finds.

4] Update your anti-virus and anti-spyware utilities definitions, then reboot into Safe Mode, (guidance here;
http://www.bleepingcomputer.com/tutorials/tutorial61.html#winxo and hint: Use the F8 method), and run both utilities one after the other in Safe Mode. Delete everything they find.

5] Reboot back into normal mode, and re-try your email program. If all is well, turn System Restore back on.

Do this soon because, if your computer is acting as a zombie and sending spam out, your ISP will soon disable your email account.