In a lot of big coporat-ey places, there is a very restrictive retention policy due to legal reasons.
If your stated IT policy is that "all mail is automatically deleted after 3 months" then that reduces your risk to discovery procedures. And since it is a published, global policy, you cannot be held in contempt of court of prosecuted for destruction of evidence.
I actually have a very hard time believing that a court is going to find that a company that only retains emails for three months isn't doing something like engaging in spoliation.
On the flip side I have never worked at a company that did not have retention policy. This includes startups and large corporate companies. Retention policies are pretty common.
If your stated IT policy is that "all mail is automatically deleted after 3 months" then that reduces your risk to discovery procedures. And since it is a published, global policy, you cannot be held in contempt of court of prosecuted for destruction of evidence.