The orange border is a sign of spam.
Akismet, the spam filtering plugin, used to be the only reason for this orange border, but on the current versions, the error with the orange border occurs for multiple reasons.
One of the reasons is the use of comment blacklist. Check your comment blacklist to see if you have words used in the problematic form submission.
Also, Contact Form 7 3.1 has introduced nonce verification for contact forms. If the verification fails, Contact Form 7 treats the form submission as spam. This can occasionally cause false-positives when using cache plugins. When an old cache file remains for longer than 24 hours, nonce verification will fail, and you’ll see an error with an orange border.
To solve this problem, make sure you have the appropriate garbage collection (clean-up) settings in the cache plugin. The cache should be emptied frequently (less than 24 hours). The configuration varies in different cache plugins. Consult your cache plugin’s support for details.