This was discovered at the same time as T137160. When attempting to send mail locally using:
$ echo -n "Test" | mail -s Test root
The mail is not delivered and the following message appears in /var/log/exim4/mainlog:
2016-06-06 22:58:34 1bA3Te-00012B-TM <= root@wmflabs.org U=dpatrick P=local S=578
2016-06-06 22:58:34 1bA3Te-00012B-TM => :blackhole: <root@jobs.security-tools.eqiad.wmflabs> R=system_aliases
2016-06-06 22:58:34 1bA3Te-00012B-TM Completed