Email Retention Practices/Policies

Looking for a good resource for common/best practices for email retention policies.
I am currently in the process of trying to develop new Email retention policies. Currently the different divisions are handling this in a variety of ways .I have to bring hem all together under one policy. The more I look for industry best practices the more I realize that there are still a lot of companies that do not have a retention policies specific to email.

There is so much to consider:
Legal requirements
Backup and storage
PST file management
Sarbanes Oxley
Instant messaging
Etc, etc?

Is an email archiving solution really the best way to go?

If anyone of you has gone through this process, I would appreciate any resource or insight on this subject you may have.

Thanks,

LHBF