Hack? – anonymous mails from my website

You need to start working your way through these resources:
http://codex.ww.wp.xz.cn/FAQ_My_site_was_hacked
http://ww.wp.xz.cn/support/topic/268083#post-1065779
http://smackdown.blogsblogsblogs.com/2008/06/24/how-to-completely-clean-your-hacked-wordpress-installation/
http://ottopress.com/2009/hacked-wordpress-backdoors/

Anything less will probably result in the hacker walking straight back into your site again.

Additional Resources:
Hardening WordPress
http://sitecheck.sucuri.net/scanner/
http://www.unmaskparasites.com/
http://blog.sucuri.net/2012/03/wordpress-understanding-its-true-vulnerability.html

but am still not sure this is a hack or not

Please scan your site with online tools such as Sucuri and note that what Esmi has noted is very good advice. I would advise the same, but also take down both the site and the email before the slightest chance my site would infect even one site visitor with malware.