rosetta-poimport hits an "AssertionError: No To: header"

In rosetta-poimport.log we started seeing a lot of assertion errors like the following:

Traceback (most recent call last):
  File "/srv/launchpad.net/production/launchpad/cronscripts/rosetta-poimport.py", line 35, in ?
    script.run()
  File "/srv/launchpad.net/production/launchpad-rev-8325/lib/lp/services/scripts/base.py", line 244, in run
    self.main()
  File "/srv/launchpad.net/production/launchpad/cronscripts/rosetta-poimport.py", line 26, in main
    process.run()
  File "/srv/launchpad.net/production/launchpad-rev-8325/lib/lp/translations/scripts/po_import.py", line 134, in run
    simple_sendmail(
  File "/srv/launchpad.net/production/launchpad-rev-8325/lib/lp/services/mail/sendmail.py", line 142, in simple_sendmail
    return ctrl.send()
  File "/srv/launchpad.net/production/launchpad-rev-8325/lib/lp/services/mail/sendmail.py", line 256, in send
    return sendmail(self.makeMessage(), self.envelope_to, bulk=self.bulk)
  File "/srv/launchpad.net/production/launchpad-rev-8325/lib/lp/services/mail/sendmail.py", line 320, in sendmail
    assert 'to' in message and bool(message['to']), 'No To: header'
AssertionError: No To: header