Question : Problem: Email message retention

Does anyone have any existing policies on how long to retain email messages? We have a retention policy for hard copy materials, but not emails. I'm not even sure where to start with this, so any policies you may have would be extremely helpful in my quest. We need to make sure we don't keep information longer than legally necessary and also, many of my users email boxes are reaching the limits I have set.  A retention policy would help in keeping storage space to a minimum.  Thank you in advance for your help.

Answer : Problem: Email message retention

What mailserver and clients are used?