How do I find email address for a contact whose name and partial email address i know(it’s not in my contacts list, although i’ve emailed him before?

Hi everyone, I have a mate and we email each other to keep in touch. I rely on my contact list (windows Mail) to find his address, I type in his Christian name and normally up pops his name and email. This morning I carried out the same process, however no name and address appeared, Can any one suggest an alternative method to obtain the address?

Kind Regards

Colin