MediaWiki (as configured by Wikimedia) imposes rate limits on emailing other users at 20 mails/user/day. Unfortunately, the limit looks to be per wiki currently. That gives users an option to bypass that limit easily, by emailing from multiple wikis. Since most wikis we operate are connected via centralauth, the wiki you decide to do emailing from doesn't matter much (it only affects language of the footer).
I propose switching to user-global, which will enforce the current rate limit at all wikis combined (so if you send 10 mails from enwiki, 10 mails from enwiktionary, you wouldn't be able to send another 10 from enwikiversity).
This is likely User-notice worthy.