Deleting email can sometimes be construed as destruction of evidence. And having email can sometimes win you court cases.
The only kind of email I delete is spam, everything else gets archived. Just a heads up, in case anybody wants to send me self-incriminating email :)
From what I know, the way this is addressed is through policy. For example, company sets policy of retaining 3 months of email. Company follows this policy. Then, email that is deleted that is more than 3 months old, is not considered destruction of evidence.
Anyway, there's too much stuff that isn't spam but has only momentary utility to keep everything. For example, automated notices are usually very high volume and always no longer useful within 12 hours.
We have annual training on PIPA and FOI (Canada) and it's stressed any e-mail, written notes or any form of recording are fair game for anyone to request seeing. Even blank lines on a notepad should have a line and initialed, all pages should be numbered sequentially.
You can destroy notes and delete e-mail in fact it's probably a good idea since the less ammunition there is the safer you are.