[Plugin: Contact Form 7] [Red Box]: Failed to send your message.

Contact Form 7 reports the “Failed to send your message …” error in a red box when anyone tries to submit a form. A command-line Mail User Agent (mutt, in this case) can send email on the host system. The Configure SMTP plugin is installed and can send test emails successfully. Contact Form 7 emails worked when the site was first configured; the Contact Form 7 configuration has not been changed.

I would love to receive suggestions about how to track down this problem. I have limited experience with WordPress and PHP (have relocated and installed one site, and fixed a few easy bugs in PHP custom code along the way). Extensive troubleshooting experience in other environments. My testbed system shows the same symptoms, so I can probably use site-breaking techniques to track down this problem.

Some kinds of replies that might help:
Pointers to outstanding articles about PHP/WordPress debugging
Pointers to discussions of how sending mail via PHP/WordPress works
instructions to “add this code to that place and see what happens”

Thanks