SQL & PL/SQL :: Unable To Use UTL_MAIL

i used UTL_SMTP to send email. it works fine and send email. but when i used UTL_MAIL, its throwing below error

Error at line 1
ORA-29278: SMTP transient error: 421 Service not available
ORA-06512: at "SYS.UTL_SMTP", line 20
ORA-06512: at "SYS.UTL_SMTP", line 96
ORA-06512: at "SYS.UTL_SMTP", line 138
ORA-06512: at "SYS.UTL_MAIL", line 395
ORA-06512: at "SYS.UTL_MAIL", line 608
ORA-06512: at line 2;

i ran below

begin
utl_mail.send('abc.def@xyz.com',
'abc.def@xyz.com','Testing UTL_MAIL Option','blah blah blah');
end;

if utl_smtp is working, means SMTP server is configured properly. then why am i not able to get email using UTL_MAIL ?

utl_mail hav public grant.

also i smtp_out_server has value in v$parameter :

select * from v$parameter where name = 'smtp_out_server';