The book "Designing Embedded Hardware" in the chapter "9.3. Old Faithful: RS-232C" mentions that emails are still sent in 7bit char set because of RS-232C:
It's also not unheard of to see RS-232C systems still using 7-bit data frames (another leftover from the '60s), rather than the more common 8-bit. In fact, this is one of the reasons why you'll still see email being sent on the Internet limited to a 7-bit character set, just in case the packets happen to be routed via a serial connection that supports only 7-bit transmissions.
How can I confirm the observation?