E-mail Hacked

[Serendipiter]

I would appreciate some help with trying to determine how my e-mail is getting hacked, or, at least, how to prevent it.
I have changed passwords each time and the last password used was very secure (the last password used was something like bHWAs5e@?Ap6UM#d&QQm_T38k5). Once hacked, the e-mail address is being used to send automated e-mails.

- E-mail browser is Mozilla Thunderbird
- Operating system is Windows XP (will not be upgraded until certain applications get modified to run on W7 or W8)
- Malwarebytes and Spybot do not detect any malware
- E-mail is accessed only on 1 desktop computer by 1 person using Thunderbird (as far as I know)

Does it seem likely that there is malware on my computer?

 

[spamjim]

Are you certain that your account is actually sending mail or is your email address being spoofed?

https://en.wikipedia.org/wiki/Email_spoofing

 

[Serendipiter]

I never received (or noticed) receiving an e-mail notification from Tek-Tips about the reply from spamjim on Dec. 1 thus my slow reply.

The way that I knew that my e-mail account was sending lots of mail was that the mail host server automatically changed the password on the account (assuming a hacker) because outgoing e-mails exceeded the host server's (programmed) rate limit of outgoing e-mails. I also spoke to a customer service representative of the server company about it.

 

[spamjim]

There is not enough information to answer your question about malware on your computer. It is possible that your password was sniffed externally if you are not using a secure connection to your mail server. Your server support staff can confirm your connection is configured securely.

There is also a possibility that the exploit was on the server's end.

 

[Serendipiter]

Thank you for your input.