Email Archiving Regulations

I am a network administrator for a credit union in Texas. I have been tasked with finding out what the regulations are regarding email archiving for our credit union. Specifically, I need to know:

1. What type of email needs to be archived? (i.e. All email, only those that contain business related or member information, etc.)
2. How long do we have to store the email?
3. How accessible does the archived email need to be?
4. Etc., etc.

Any information regarding these regulations, or where to find these regulations, would be greatly appreciated.

Thank you.

Matt