Thanks for the reply. I understand about multipart emails. Unfortunately my users don't. I know you can select a preference in mail clients whether to show html or plain text emails, but again my users don't. I'm thinking that from an ease of use perspective it'd be easier for them to set that preference in there WordPress profile. Certainly a lot easier to educate users that way.
As you guessed I've got as far as setting up the options form and retrieving said options. The problem I have is in mapping that option to the email being sent. As far as I can tell the plugin hooks into PHPMailer, which is only passed $to, $message, $headers and $attachments from wp_mail(). To make the plain text preference work, I need to identify the $to variable and match that to the user profile, but at the moment I can't get the content of the $to field.
I think the $to variable in the PHPMailer class is private, so can't be accessed there. I had a go at filtering wp_mail but didn't get very far. What would be really nice is if the plugin could take the userid of recipient as an input, but that would require re-writing the wp_mail function, so again I don't think really feasible. Right now I'm just looking for some other ideas to try, as you obviously know the plugin inside out you're probably my best bet.
I already patched the php for the string replacement. I also added a string replacement for %subject% in the wpbe_send_html function, as I wanted to include the email subject in the email body. I would link to the patched file for you, but can't access my server at the moment! I'd also suggest creating a new filter in the string replacement function, so users can define their own replacements.
Thanks again for your help.