Sender notifications - disable? customize?

[MRob]

Are there best practices advice about sender notifications?

Looks like default is to have notifications ("warnings?") enabled for 
attachments with banned file extensions. Doesn't this risk backscatter 
problems when sender address is forged?

I cannot find a way to disable them - tried setting these all to 0 but 
no luck: $warnbadhsender $warnbannedsender $warn_offsite

I'm having a very hard time finding documentation on any of those 
settings, what exactly they do - where can I find that?

Also, is there a place to customize the message body (and possibly 
headers) for these notifications ("warnings?")?

Thank you for your great product.