If you need the email to appear as if it was sent by a user and when replied to reply back to that user's actual email address then you can make use of reply_to.
user_name = 'Some Guy'
user_email = 'some.guy@whatever.com'
admin_email = 'no-reply@yourdomain.com'
from_email = '%s <%s>' % (user_name, admin_email)
reply_to = '%s <%s>' % (user_name, user_email)
mail.send_mail(from_email, to_email, subject, body, reply_to=reply_to)
This way to the person receiving the email it will appear that it came from user_name and if they reply then their reply email will be sent to user_email.
The main negative of this approach is that if someone looks closely at the address they received the message from they will see the admin email address but most email clients show the name more prominently.
On the plus side you can send as any email address and most people would never notice it came from an admin email address.