Sending unique emails to new customers

Hi,
Iwas wondering if anyone knows a a way or a program that can fix my situation. Simply put, I want to sent unique emails to new customers. This means when customers buys a plan on my website he/she should receive an username and password but everyone who subscribes should receive a unique username and password. Does anyone have a clue how this could be done?
thanks for everyone helping already.

Sincerely,

Hierism