Sending from multiple email addresses using POP

Hi,

I have an app with many users who need to send and receive emails. My client does not want to have a local smtp server (if he can avoid it :). I can receive emails on a per user basis using Net::pop. What has me stumped is how to send email from different email accounts(but same domain)

When I try to send if the FROM: does not match the smtp login account the mail server does not send it. Since Rails has only one app wide smtp configuration is there a way to use a per user login/auth to send email as well?

TIA,

bakki

PS: Our app will be deployed on a separate VPS for each of my client's customers and each customer is a multiuser env.