Why did I receive email from this site on a holiday such as Succoth when we are not allowed to use any electronics?
A Jew is forbidden to send (or use a computer to read) email on Shabbat. Further, asking a gentile to send you something specifically on Shabbat is forbidden; you can't ask someone to do for you what you can't do yourself.
However, if the action was taken on Shabbat but you didn't specifically ask for it to occur on Shabbat, that's fine. See this parallel case about physical mail, and this one about automated bidding in an online auction that will conclude on Shabbat. I presume that you did not ask the sender to send this email to you on Shabbat (or in this case Yom Tov).
Further, this is all about specific, intentional actions. The email you received is likely to have not been of that type -- either it was an entirely automated message, like the weekly newsletter, or it was a message triggered by some state on the site (like inbox messages, a bounty about to expire, or a top-new-user notice). An automated process cannot form the intent to act specifically on Shabbat/Yom Tov.
Many Jewish organizations that have automated email take steps to avoid it being sent on Shabbat out of concern for the appearance that sending it entailed a Shabbat violation. While Mi Yodeya is a site about Judaism, neither it nor Stack Exchange, our host, is a Jewish institution. It is not surprising that automated email from a non-Jewish organization is sometimes sent when melacha is forbidden.
The server is not shut down so the email may have been sent before Yom Tov but not delivered just as snail mail put in the postal box before Yom Tov may be delivered on Yom Tov. Another possibility is that someone in a timezone where Yom Tov was already over sent the email.
Your responsibility is to not use the computer on Yom Tov or Shabbos. What other people do is up to them.