Spotted while browsing https://shinken.wmflabs.org/problems , it seems exim on tools-mail has a too many blocked messages in its queue.
The statsd metric tools.tools-mail.exim.queue.length over 24 hours:
That matches the frozen metric, so probably messages that cannot be delivered.
And for tools.tools-mail.exim.queue.size over a week (I have no clue what the size is, maybe that is bytes):