Can you post some specifics so we can try and replicate and ultimately fix please?
Fundamentally at present the plugin is broke whilst the text option does work it means some pretty nasty looking emails
the switch to wp_mail would explain it. Are you setting the wp_mail_content_type and headers correctly? Most likely the order.
One way would be to compare the headers from the one using mail() and one using wp_mail()
Try setting the from email as [email protected]
anyone? suggestion very much appreciated