I'm posting this as feature request for two mods (APTL and AUT) as both have it in new version. I'm not sure if mail sending upon action (mentioning, thanking, liking, etc.) is in free version, but I noticed this here (were it is in full version).
Anyway, I think that it would be good if mail sending could be turned off by default in userCP, so users need to turn it on if they want to receive emails about actions. Or at least that we have option in ACP to have this turned on or off by default for all our users.
I don't know how many users would like to receive emails about actions, but I think it is much better for larger sites and majority of users to turned this on if they want to receive emails. For example, I received e-mail that I was quoted here, so I turned it off for all actions for myself. Users who wants e-mail notification, they could turn it on themselves.