In the U.S. (relevant because it is home to the Wikimedia Foundation), you can sue anyone for any reason at any time. You might get immediately dismissed, sued back ("abuse of process" or similar), or something along those lines, but there is nothing structural that stops you.

The structural reason that you can't sue email is that email is not an "anyone", it's an abstract concept. How would you even e.g. notify "email" that it is under litigation?

Neither email not SMTP are legal entities.

Wikipedia (or more precisely, the Wikimedia Foundation that owns it) is.