I recently had a problem with Tophost hosting and WP Contact Form 7
the problem is from Tophost, it blocks every mail sent without that parameter.
I have resolved by hacking the wordpress core:
search the line with
public $Sender = '';
at about line 97, this is an empty string, put your email instead and save the changement.
now you will receive email, phpmailer automatically set the -f parameter when this string is not empty
Would be great to put a hook filter to control the value of that variable because at every wordpress upgrade you have to rewrite your email
so I propose to do add a filter here, or maybe a constant to define in wp-config.php