Send mails use the same source IP across multiple servers

[Ram]

Hi

I have a slightly OT question
If I have to use a single IP for a sender domain to the internet, but 
yet the mails may get sent from different servers
What is the best way for doing it

The requirement is because the volumes are too large for a single 
machine to handle but the client still wants to send the mails using a 
dedicated IP




Thanks
Ram