Please see (Note: 753845.1 – How to Perform a Meaningful SMTP Telnet Test to Troubleshoot Java Mailer Issues), the same error is reported in this doc.
Note: 1054215.1 – How to Check if the Workflow Mailer is Running
Note: 415516.1 – How to Check Whether Notification Mailer is Working or Not
Note: 831982.1 – 11i/R12 – A guide for troubleshoting Workflow Notification Emails – Inbound and Outbound
Note: 1012344.7 – Notifications Not Being Sent In Workflow
Note: 560472.1 – Workflow Mailers Not Sending Notifications