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

This error message tells that localhost, the same server where you have your Oracle database running, does not have an SMTP server up and running.

Means this error occur when SMTP SERVER IS NOT REACHABLE so it is not oracle error.
when UTL_SMTP package not able to connect SMTP server then above error will occur.

So first need to check without involving oracle we are able to connect smtp server through TELNET command

for eg:

C:\>telnet SMTP_SERVERNAME port (default port 25)




A Qestion:- do we need smtp and oracle on the same server ?
===========================================================

Answer:-
===============

No, its no need to, smtp and oracle on the same server.

You first have to check whether you are able to contact the email server without involving ORACLE.
you need just connection with SMTP server

C:\>telnet SMTP_server port
Connecting To SMTP_server Could not open connection to the host, on port 23: Connect failed

You will see the above error it means system is not able to connect smtp server that is why the above error occur.

after successfully connect to SMTP server , then try via oracle database.