I would like to present this scenario as the question:
My company has a POP3 Id.
I dial into my provider's network using PPP over a serial connection. Our incoming mail is spooled at the provider's POP or IMAP server, while
outgoing messages are to be sent via SMTP. We don't have a domain
name of our own, so everything has to use one address.
My Requirements :
1. Sending mail between various local users must be possible.
2. The outside world must see the local users' ISP mail addresses, not
the local ones.