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Where is the site?

Here is a basic standard contact form you can test:
https://www.illumedic.se/test/

Thanks for any assistance and input you might have.

How have you set up the form in the Form section in Contact Form 7’s admin page?

It’s just the default setup, without any changes what so ever, and includes e-mail address and all the form info needed.

I run Contact Form 7 on multiple sites (20+) without any issues, conflicts or otherwise, so you can be assured it’s set up correctly in the form section.

As a note I have also tried switching to the Twenty Twelve theme which made no difference.

I’ve also tried stripping htacces file of everything I had added there, no help.

I also checked file permissions of the contact-form-7 folder and files (755) – no difference.

Have you tried deactivating all other plugins?

Yes, as mentioned in the first post.

Do you use Comment Blacklist feature?

Fantastic! You solved it!

That was it. There was a long list of keywords in the Comment Blacklist.

Takayuki, Thank you!