Changing Password is Not Exactly A Solution.
This is a malware problem. If your computer has been compromised (not your email), and you are either in your email program (outlook for example), malware accesses your contact list, which is not protected, and then opens up a port and sends.
Changing passwords won't help because you have already opened up and are using the email client. You can keep changing the password and the malware will still be able to send emails.
This problem is less likely if you use a browser for accessing your emails. Yahoo or Gmail for example .
Clean your computer with Spybot, and CCleaner etc.. (tons of info already on Cnet), and then consider using just a browser based email.
Of course there is/are no perfect solutions to preventing breaches. Another option is to set up a NEW email account and have separate accounts for you need to provide email addresses online, vs. the email that you give out to friends and business associates etc..