There are many reasons why a particular email does not arrive; that doesn’t mean this plugin is not adding the email to the queue, nor that CF7 isn’t trying to send it. No additional mail (including the built-in Mail 2) will send if the first email send fails, other plugins could conflict with this one if they modify CF7’s behaviour at all, and any normal misconfiguration of a message can prevent it sending. As the description states, the extra mail “does not include the automatic configuration error detection that the default mails have.”
Have you tried creating a test form and copying the details from the problematic form’s additional mail into the test form’s first Mail, to automatically detect errors? Have you tried visiting https://www.mail-tester.com/ or a similar service, and putting the address they give you in the additional mail’s To field? It will give a good indication of whether it’s being sent at all and, if so, why delivery could fail.
I’m sorry but I’m not convinced this plugin itself is the cause. I just created a fresh contact form that only used Mail and Mail 3, both emails arrived fine. I’m using the same v1.0.0 of this plugin as you, and v5.1.6 (the current version) of Contact Form 7. What version are you using?
