I had a client that reached me with a problem of suddenly contact form from his website stopped sending emails.
Site is hosted on GoDaddy, and plugin used to send form is Contact Forms 7.
Funny issue here is that the form, after pressing the button 'Send', actually returns a green 'message sent' message to the web user.
Checking in the logs showed that the message was actually sent using local mail delivery / default. OK.
Trying to install WP SMTP and send message through SMTP, I found out that the ports 25, 465, and 587 are filtered out for sending email to remote SMTP server.
Sending test email using WP Test Mail is successful. Each and every time message reach the remote recipient.
After debugging and analyzing each and every part of sending email, the problem happened to be the body of the message itself. It contained manually entered full domain name ( https://domain.com ) of the user. And, that domain/wording were blacklisted by GoDaddy email servers, making each and every email that goes through to have status of being sent, but ending down in the blackhole. Removing just the mentioned URL ( https://domain.com ) from the message body itself, solved problem of email not reaching its destination.