Both. - by September thirtieth means you want them to send an email before this date, - on September thirtieth means exactly on this date

Thank you, Ella.

Both questions are possible but there is a difference in meaning. The first question 'Can you send me an email by September thirtieth?' means that you would like the other person to send the email up until September thirtieth .This means you can send the email anytime before this date, or no later than this date. The second question- 'Can you send me an email on September thirtieth?' means specifically only on this date. Not before or after this date- only on this day. Hope this helps :)