If a company is served with a subpoena, and it in turn emails all of its employees asking for any related documents, does the email carry the same powers as the original subpoena? Jurisdiction: US federal law
No. The e-mail has none of the powers of the subpoena. You may elect to ignore an e-mail from your boss (and get fired) but you may not elect to ignore a subpoena.
We don't have a lot of background here, but the more important question, probably, is the extent to which the employees are obligated to comply with the subpoena itself, and what penalties may apply to employees who refuse.
If a company is served with a subpoena under Rule 45(e), contempt of court is the only sanction, and Rule 45(e) only allows a finding of contempt against "a person who, having been served, fails without adequate excuse to obey the subpoena."
That service of the subpoena is a critical due process prerequisite, so it is not likely to be maneuvered around. So if an employee hasn't been served, my conclusion -- having never seen a case like this -- would be that the employee is not legally bound to comply.
does the email carry the same powers as the original subpoena?
Yes insofar as a non-compliant employee could be sanctioned for obstruction of justice. Employees are not allowed to disregard or to try to evade/overcome the company's email under pretext that they are not individually named in the subpoena.