I have faced this problem for several of my client's sites.
As I was looking for the problem. I saw the SMTP is working just fine. Hosting is not blocking the port.
One thing common was the CF7 integration with the Google Re-captcha V3. After removing the integration of Google Re-Captcha V3 from CF7 the forms started to work again.
Obviously this is not a solution just a turn around.
So, to stop spamming I have activated Akismet Plugin which is better than the Google Re-Captcha system.
Aksimet will stop the comment spam and to add the tag into your CF7 forms, please follow the guidelines in here: https://contactform7.com/spam-filtering-with-akismet/