It appears the dns message (including page not found and blank page) was
a symptom of trying the email test page too often resulting in more than
the allowed number of simultaneous processes (20 are allowed on the
shared server).
However, then I got a further solution:
The apache timeout on the server was set to 3 seconds, rather than the default 300. This was likely to combat a ddos attack from earlier, but never changed back to its default.
Basically, what was happening was when the mail() function was called, sometimes it was taking longer than 3 seconds to process, and since no data is sent to apache during that, it reaches it's timeout and stops.
We've changed this back to the default of 300 seconds (5 minutes) and now it isn't stopping output when it needs to send an email.
Thank you to all who took an interest in this.
John
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