It was great ever plug-in that we used but we realised a bug today!
I was working great at the begining but later today gives error failed to send email cause of Akismet spam. So I deleted Akismet but it gives same error. I thought first contact form 7 is not working properly, but when i research, the problem is Configure SMTP. When I change that plugin with alternative one, it works like a charm. Somehow, after sent many emails, its not sending more and Contact Form 7 gives that error.
But I am unhappy because I like Configure SMTP gmail function. I hope you solve that problem soon.