Hello, Sean
Please read this article – https://contactform7.com/faq/i-see-green-border-response-but-i-never-receive-mail/ and check the log of your mail server, it could give you some clues.
You mentioned that the problem persists with another plugin too, which could indicate that something is not right from the server side. We recommend that you contact your hosting provider to investigate the situation further.
Kind Regards, 8theme team
