UTL_SMTP send mail wield problem????

we have ORACLE 10.2.0.4 with Redhat AS 4.4 on DELL server.  Recently we are test "UTL_SMTP" on ORACLE PL/SQL program and tried to send MAIL to customers.
 

The problem is this send MAIL program always one time work and second time not work (loop).  I don't think problem come from ORACLE bug or any ORACLE relate problem.  If I copy database and put on anyother server, it work correctly.
 

Does anyone have suggestion?
 

====== sample event happen  ===
 

Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - Production With the Partitioning, OLAP and Data Mining options SQL> @mailbody
 31  .

SQL> /
Declare
*
ERROR at line 1:

ORA-29278: SMTP transient error: 421 Service not available
ORA-06512: at "SYS.UTL_SMTP", line 21
ORA-06512: at "SYS.UTL_SMTP", line 97
ORA-06512: at "SYS.UTL_SMTP", line 139
ORA-06512: at line 15

SQL> /
PL/SQL procedure successfully completed.
 
 

SQL> /
Declare
*
ERROR at line 1:

ORA-29278: SMTP transient error: 421 Service not available
ORA-06512: at "SYS.UTL_SMTP", line 21
ORA-06512: at "SYS.UTL_SMTP", line 97
ORA-06512: at "SYS.UTL_SMTP", line 139
ORA-06512: at line 15


      

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