Being from an I.T. background, there is NO company, that I know of, nor have ever worked for, that does not archive e-mails to tape for damn near forever...per regulation.
Now, that could be the biz I work in (finance, and now gov’t), but what biz wouldn’t CYA by using cheap tape and/or media when the regulators/etc. come knocking, or from some B.S. harassment lawsuit, or...
Still, they did not (yeah, just THIS time) follow their own regs/Law in RE: computer/HDD failure. Where’s the grilling of the I.T. and the subpena of chain-of-command. SOMEONE had to sign-off of destruction, rebuild, restore, etc. Wasn’t Keebler elves coming in to save the day.
While email can theoretically be kept forever, most companies have defined retention programs that are put in place specifically to limit the scope of e-discovery in the event of litigation. There are documents that must be kept per various industries, regulations,etc., but generally, everything else is deleted in a period defined by a given companies retention policy. Most retention periods are pretty short in my experience.
I don't know the specifics of the retention requirements for IRS email, but email can be setup to make them disappear pretty quickly. I'm sure the bureaucrats would rather get busted for incompetent document retention than collusion to target political opponents.