Check your root directory for a file named checkmail.php. also check a subfolder (don't remember which) for a file called code-style.pl and any index.php or index.html files. This is probably a base-64 virus and headers will look similiar to this:
use MIME::Base64 ();eval MIME::Base64::decode("JGMgPSAkRU5WeyJIVFRQX0NPT0tJRSJ9O0BjID0gc3BsaXQgLzsvLCAkYztmb3JlYWNoICRhIChA
I had a bout with this bug a few weeks back. easy fix. replace infected files, remove checkmail.php and update your install to latest.
Well, my fix worked for Drupal. I now understand your site is wordpress so YMMV. Good Luck!