Greetings,
Unfortunately this is not possible. The program is currently for a single administrator and it is not possible to customize the "Send From" email to customize an autoresponder for multiple users.
Your only option is to resell ListMail to your clients so they have their own copy (and you can earn some money) I can offer additional memberships at a discounted rate:
$120 for 1
$216 for 2 (10% off)
$288 for 3 (20% off)
$360 for 4 (25% off)
$420 for 5 (30% off)
$468 for 6 (35% off)
$500 for 7 (40% off)
$600 for 10 (50% off)
$1080 for 20 (55% off!)
Please let me know if you have any more questions!
Regards