After all the tweaking and testing yesterday the daily email didn't go out overnight. Testing this morning also failed so it wasn't an issue recreating the cron jobs. I may have to look at alternative solutions.
It's so frustrating that something I built and am quite proud of is hampered by something as arbitrary as rules on a spam filter - an outgoing one at that - that no one appears to have any control over. Because the antispam setup is outsourced the support guys are not even able to tell me what rules are being triggered, it obviously just doesn't like the mail format. Something just looks like spam and the system takes objection.