I am glad that I don't work for $BIG_CORP, I have all of the e-mails I have ever sent or received from a customer store in a folder for that customer (my inbox is in effect my to do list, the only e-mails in there are ones I have to respond to, either by reply or by doing work if they are from one of the work tracking systems.
The reason I do this is for the following situation, this is an actual example, names have been removed to protect the guilty.
1. I discover a bug in the minor piece of software that I develop where a common mistake in data input can result in a customer not being billed correctly. I trace the history of the bug and notify all affected customers that the bug exists, the conditions that it occurs under and why it is going to affect all of them (the input is a file coming from a source they all share) and that the fix is free as per their support contract.
2. All but one of the affected customers ask for the fix straight away, the other says they don't think it is important so they don't want it.
3. Fast forward one year customer who doesn't get the fix realises that their bills are coming out wrong, raises all sorts of hell with my boss about the buggy software that they are using, I forward their e-mail saying that they don't want the fix and suddenly they go very quiet.
Without my e-mail history it would be my word against theirs. Finally almost all of my projects last longer than 90 days, how do you keep track of what was agreed (most importantly agreed to be excluded) at the start if all the e-mail trails are gone.
I am left wondering what dodgy things $BIG_CORP are up to if they think e-mails over 90 days old are a legal risk.