letterhead required for recommendation emails sent electronically?
I would not worry.  It's traditional to use letterhead for electronic recommendation letters, but it's not really essential.  The admissions committee is very unlikely to care whether letterhead is used, and it should not have any effect on their decision.  In the unlikely event that they have questions about the letter's authenticity, they would contact the writer directly (e.g. by looking up their contact info on the university's web site).

If they haven't sent the letters yet, you could let them know that letterhead is customary and suggest that they use it if convenient.  If the letters have already been sent, then I'd just ignore it.  It's not worth the trouble of resubmitting.