Mailgun, as http://www.mailgun.com/ says, is a powerful API that can send roughly 1000 emails in one API call. Mailgun provides free use of upto 10k emails per month and loads of accounts too in your custom domain. Hence, it is a scalable option for a person/group hosting a wiki, and want to send emails for free.
Currently, Phabricator has adapter to use Mailgun API ( see : https://secure.phabricator.com/book/phabricator/article/configuring_outbound_email/#adapter-mailgun ). There is one for wordpress too https://wordpress.org/plugins/mailgun/.
It would be great to have an extension that can make use of Hook::AlternateUserMailer, that can use the Mailgun API feature to send out emails. Currently, we can only use its service by SMTP configuration, which is lot less slow than the API call, which mass-messaging services like phab and Mediawki requires.