Creating Mail Relay server for ISP customers

Hi there

I work as Network Administrator in Internet Service Provider. What I want to create for our customers is Mail Relay server, only on few IPv4 prefixes that we recently get from as a LIR. Some of this customers use some free webmail emails from different ISP, using POP3 and SMTP with Email clients, and those providers allow to use their SMTP server only from their own IP addresses.

I was thinking to block port 25, and allow from our IP addresses to use our Mail server only as Mail Submission Agent on port 587, but without TLS and authentication. Also we will create some custom scripts to monitor 587 port usage for email limits, to fight sending Spam from our customers.

I installed Virtualmin from install script on fresh Ubuntu 12.04 server. I tried with few settings in Postfix according to some topics from this forum and some other site, but without success.

One of the errors is this one:

NOQUEUE: reject: RCPT from unknown[]: 554 5.7.1 <unknown[]>: Client host rejected: Access denied; proto=ESMTP helo=<[]>

Maybe some one will suggest using different solutions.