SMTP Error. Getting below error while trying to send email in XXXXXXX instance

Getting below error in EBS R12.1.3 in Oracle Database 

SMTP Error. Getting below error while trying to send email in XXXXXXX instance

Oracle Error in Send Mail prog--ORA-29277: invalid SMTP operation

ORA-06512: at "SYS.UTL_SMTP", line 82

ORA-06512: at "SYS.UTL_SMTP", line 540

ORA-06512: at "SYS.UTL_SMTP", line 525

ORA-29278: SMTP transient error: 421 Service not available 

It can be below different issues.

1. You don't have enough privileges.

2. SMTP server not given properly.

3. Check WF config to validate SMTP server.

4. Firewall ports are opened for the SMTP mail. Use command prompt to send email.