This update allows you to use auth_smtp with smtp_proxy. It basically
provides a transparent SMTP proxy service to the back-end system. You can
then use one SMTP server (Haraka) to forward your emails to different
back-end servers based on your username. Especially useful if you are
using one IP-address and for example, two DavMail servers. This change
directly complements what Perdition (Mail retrieval proxy) does for POP
and IMAP.