Could not start mail function.

I'm getting the following when I try to send an email from Chronoforms:
​[h4]Notice[/h4]
Could not start mail function.
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I know that my email is working because I can send a test message from the General Configuration page.

Anyone have any thoughts on where to look? I'd really like this working...

Hi threecats,
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Please try enabling site Error Reporting temporarily and see if you get a more helpful error report.
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Bob

Unfortunately, there didn't seem ti be anything I could see in the logs.

Hi threecats,
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If you PM me a SuperAdmin login I'll take a look - but I am not an expert on CFv7 :-(
​
Bob

Hi threecats,
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I suspect the problem is with the SMTP mail settings. If you test with SMTP selected the test email fails. I'm guessing that CFv7 defaults to SMTP - though I can't find a setting to confirm that.
​
Bob