As the trusted IT provider for your clients, one of the 'perks' of your job is that you’re the defacto go-to authority for all manner of (sometimes fairly dry) topics.
Email retention policy quite possibly ranks high on the 'dry' scale, but it’s a topic both you and (and they!) really do need to be aware of.
The mailboxes of your customers are rapidly growing treasure troves of useful, important and sensitive information, and that will continue to be the case for years to come.
This makes them ticking time bombs if not cared for in the right way.