choosing random ip address for outgoing smtp connection

[Artem Bokhan]

Hi, can someone give me a hint how to randomly spread outgoing smtp 
traffic over pool of ip-addresses?
As I understand it's necessary to create several transports and use 
transport tables, but I do not see a way how to make lookups truly random.

smtp1      unix  -       -       n       -       100       smtp -o 
smtp_helo_name=smtpout1.do -o smtp_bind_address=1.1.1.1
smtp2      unix  -       -       n       -       100       smtp -o 
smtp_helo_name=smtpout2.do -o smtp_bind_address=2.2.2.2
smtp2      unix  -       -       n       -       100       smtp -o 
smtp_helo_name=smtpout3.do -o smtp_bind_address=3.3.3.3