hope this help steps.
1. check if email address are inputted correctly
2. check if Server Name, SMTP/IMAP was inputted in the 3rd party application
3. check your firewall if you allowed the third party to send email.
i'm currently working on custom forms. and one of the rules is to attached a file before you send the forms. i already created a warning/question if they attached file. some of the users still continue to send the forms even if they didn't attached any. is there a way to capture that...