I found this in the logs
[2015-04-28 05:39:49,330] INFO com.google.gerrit.server.mail.OutgoingEmail : Skipping delivery of email with no recipients
[2015-04-28 05:40:32,811] INFO com.google.gerrit.server.mail.OutgoingEmail : Skipping delivery of email with no recipients
[2015-04-28 05:40:38,263] INFO com.google.gerrit.server.mail.OutgoingEmail : Skipping delivery of email with no recipients
[2015-04-28 05:44:19,863] INFO com.google.gerrit.server.mail.OutgoingEmail : Skipping delivery of email with no recipients
[2015-04-28 05:44:19,989] INFO com.google.gerrit.server.mail.OutgoingEmail : Skipping delivery of email with no recipients