SMTP won't send mail suddenly

So, I have a Windows Server with extJS customized app, one of apps features were mail responding, it worked perfectly until few days, then all of a sudden it stopped delivering emails, I tested with simple phpMailer it delivers, tested the "pickup" folder also delivers, but but won't deliver emails sent from this app.

I ran through settings, everything is as the MS official site says it should be, including Relay and everything - as I've said it workd already.

Now I really need a way to reset SMTP Virtual Server #1, I tried stoping and starting through PowerShell, tried adding second SMTP Virtual Server, and disabling the first one but it still won't go.

I can't post you code of mailer of the app cuz developer that wrote it secured it with SourceGuardian, but nevertheless it worked until few days ago.

via Chebli Mohamed