I agree, mass messaging to the alliance, or even worse, the family of alliances including wings, is a frequent problem. Some are too lazy (or don't understand) to select their own list and simply reply to a previous legitimate mass mailing by a cic and delete the messages in the thread, rename the mailing and send it to hundreds. On some worlds I'm on mass mailings of this kind even though I'm no longer in any of the alliances having already got a badge.
Limiting the number of recipients would help - then the mailings would need to be repeated of course. But if 75 were set as the limit it would help, instead of 500 (10 alliances) clogging up the system.