Randomize Source IP Addresses With Postfix

Enters Postfix version 3. It introduced a new randmap table which made this much easier to accomplish.

In main.cf, just add the following lines

sender_dependent_default_transport_maps = 
randmap:{relay1,relay2,relay3,relay4,relay5}
smtp_connection_cache_on_demand=no

Those lines have 2 effects:

Randomly select an SMTP client for sending each email
Prevent the SMTP clients from caching connections, so sending multiple emails to a single domain does not end up using the same SMTP client.
In master.cf, just add new SMTP services and configure them to bind the designated IPs

relay1 unix - - n - - smtp
-o smtp_bind_address=IP1
-o smtp_helo_name=foo1.bar.com
-o syslog_name=relay1
relay2 unix - - n - - smtp
-o smtp_bind_address=IP2
-o smtp_helo_name=foo2.bar.com
-o syslog_name=relay2
relay3 unix - - n - - smtp
-o smtp_bind_address=IP3
-o smtp_helo_name=foo3.bar.com
-o syslog_name=relay3
relay4 unix - - n - - smtp
-o smtp_bind_address=IP4
-o smtp_helo_name=foo4.bar.com
-o syslog_name=relay4
relay5 unix - - n - - smtp
-o smtp_bind_address=IP5
-o smtp_helo_name=foo5.bar.com
-o syslog_name=relay5