Database Software

I've set up process flows to execute mappings and send email when a mapping fails. However, whenever it tries to send an email, the process flow always fails with the following error:

ORA-20000: Failed to send mail due to the following error: ORA-29278: SMTP transient error: 421 Service not available

This error typically indicates that the mail host isn't available or the port is not open. However, I've set up the correct smtp_server and port parameters for the email activity. I've also sent email through a pl/sql program using the same mail host and port on the same database and didn't have any problems. Is there any additional setup in workflow or owb process flow that I need to check?

Any suggestions would be greatly appreciated!