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

Hi,all
When I try to use UTL_SMTP package to send mail,I got the following error:

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 "SEC_SYSTEM.DEMO_MAIL", line 240
ORA-06512: at "SEC_SYSTEM.DEMO_MAIL", line 118
ORA-06512: at "SEC_SYSTEM.DEMO_MAIL", line 104
ORA-06512: at "SEC_SYSTEM.MAILTEST", line 3
ORA-06512: at line 2

Then,where is the problem? Thank you in advance!

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