From a regulatory perspective, financial services is the most mature industry, having developed regulations that require formal records of all communications to monitor activity, conduct the examinations, investigations, and reviews that prevent sales practice and advertising violations, fraud, and other abuses (these include SEC Rules 17a-3 &d 17a-4, Rule 204-2 of the US Investment Advisors Act and FINRA Rules 2210, 3110, 3130, 3120 & 4511).
These financial regulations require 5-7 years of retention of records encompassing all written business-related communications, including not only email, but also instant messaging, text messaging, WhatsApp, WeChat, Bloomberg, LinkedIn, Facebook, Twitter, Instagram, and other social media usage.
Similar requirements are being implemented by financial regulatory authorities globally.