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If you don't delete them, then your e-mail archives could be used against your company.Unfortunately, the flip side might be as bad or worse: If you deleted emails or other potentially-relevant documents at a time when a lawsuit was reasonably foreseeable, then you could be hit with an adverse inference [1]. That is, the judge might conclusively presume (or instruct the jury to conclusively presume) that the missing emails or documents must have been really bad, otherwise you wouldn't have destroyed them.
And if government authorities were involved, then you could be indicted on criminal charges of obstruction of justice. (That ultimately destroyed accounting firm Arthur Andersen LLP [2].)
That's why opposing counsel can get downright orgasmic when their adversary has deleted emails or destroyed documents, and when this can be "spun" as having been motivated by a desire to conceal harmful evidence.
I used to think it made sense to adopt, and then follow, a comprehensive "document retention" [sic] policy, where documents are reviewed every X months for possible destruction. In the real world, though, people get busy, and they don't take the time to do periodic document reviews. Then, suddenly, a lawsuit is imminent, and people realize they still have all these documents that should have been routinely destroyed but never were. So they try to play catch-up with a shredding party. The belatedly-destroyed documents might be utterly harmless, but that might not matter. (A belated shredding party is what got Arthur Andersen indicted.)
RULES OF THUMB: If your organization doesn't already have a document-retention policy, here are two serviceable rules of thumb that might help (YMMV, I'm not your lawyer, etc.):
A) Keep emails and other documents for as long as you think you'll have a business need for them -- and keep in mind that sometimes the law mandates specific retention periods for certain categories of document;
B) If there's a non-trivial possibility of a lawsuit that would be even remotely related to the subject matter of the emails or other documents, then don't destroy them without first checking with your lawyer. As the saying goes, sometimes the cover-up --- whether real, or simply the product of opposing counsel's "spin" --- is worse than the crime.
[1] http://en.wikipedia.org/wiki/Adverse_inference
[2] http://en.wikipedia.org/wiki/Arthur_Andersen