Message '500 line limit exceeded' – email with attachment or newsletter

Hello,
I’ve got a little problem but I can’t find the solution for days.
When I’m testing Contact Form 7 with a simple form, everything works, I receive the contact form without problem…
BUT when I’m testing Contact Form 7 whith a form with attachments like images, or when I’m trying to send a newsletter with Wysija, the message is that everything works well, but the sender receive after that a message from the Mail Delivery System saying that the message or newsletter could not
be delivered because of ‘500 line limit exceeded’.
I know I have something to change in one php file, but I don’t know where and how ?
Thank you for your help !
I’m just looking for answers for days but without success.