Coordination of a job on multiple machines

I need some help with a architecture design. I have a service that is supposed to read a large number of customer records and send an email based on some of the fields in each record. This process can take some time to complete, so will need to be shared across multiple machines. The problem with running the process on multiple machine is coordinating, so multiple emails do not get sent to the same customer, also consider the time it takes to send the mail and get a response from the email server. I have some ideas, but wanted to know what others have done. What design patterns/architecture should I look at for comparison?

Thanks in advance.