Failed to send message (seen FAQ)

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I installed Word Press v.3.5.2–zh_TW.
And two plugins: Contact Form 7 (v.3.4.2), Really Simple CAPTCHA v.1.6.

Word Press is working well, but I can’t send message with Contact Form.(Even stopping the CAPTCHA, it’s the same.)

When I click Send, it shows RED border line “Failed to send your message”.

I search the Contact Form Website, but the FAQ of this problem is too old.(WP-Mail-SMTP plugin is no more to update for 2 years.)
How can I solve it.

Thank you~

https://www.ads-software.com/extend/plugins/contact-form-7/