Email notifications not sending when build is triggered by SVN

I have a build configuration set up with two triggers - one is a scheduled nightly job, the other is a VCS trigger (I'm using Subversion for the VCS).

No problems with the nightly job. The trigger fires appropriately, builds run successfully, and email notifications are sent correctly per notification rules.

The VCS trigger fires appropriately, and runs successful builds. But no emails are sent. We're getting the following in our teamcity-notifications.log:

xxxxxxxx) {id=24}]. Context: event='build_successful', build='Unit Testing #92'. No notification is sent 

Note that the 'xxxxxxxx' stuff is just a redacted username.

Any ideas?