Error when subscribing to newsletter

I'm recieving the following error when I try to subscribe to the newsletter from the front of end of the site.

I receive error if I'm using a triden theme or twenty ten 1.3 theme.
Would you have a fix for this issue? Do you need any addtional information to trouble shoot?

Warning: Cannot modify header information - headers already sent by (output started at /home/bt1161/public_html/velvetcloak/wp-content/plugins/e-newsletter/email-newsletter-files/phpmailer/class.phpmailer.php:586) in /home/bt1161/public_html/velvetcloak/wp-includes/pluggable.php on line 866