The SEC requires that email for any employees handling securities trading be maintained in a tamper proof archive. The purpose, of course, is to keep a trail of any insider trading discussions that may have be going on in email.
For the normal banking and non-trading services however, no regs yet. I would just offer a caution. Watch out for employees using email as a de facto document storage system. If it was me, I'd have an "acceptable use" policy that deems email as a transient resource and requires employees to move any important memos or documents to a proper file server or document file cabinet. With that in place and communicated, you could manage (and purge) your email system as appropriate to the capacity of the technology and you can manage your document storage and retention as a separate practice.