SQL & PL/SQL :: Sending Mail ORA-29279 - SMTP Permanent Error 550

I'm trying to send email using utl_mail...code is below

BEGIN
UTL_MAIL.send(
sender => 'account@test1.com'
,recipients => 'acount@test2.com'
,subject => 'Test Mail'
,message => 'Hello World'
,mime_type => 'text; charset=us-ascii'
,priority => 3);
END;

but getting following error

ERROR at line 1:
ORA-29279: SMTP permanent error: 550 <account@test2.com[/email]> No such user here
ORA-06512: at "SYS.UTL_SMTP", line 21
ORA-06512: at "SYS.UTL_SMTP", line 99
ORA-06512: at "SYS.UTL_SMTP", line 241
ORA-06512: at "SYS.UTL_MAIL", line 424
ORA-06512: at "SYS.UTL_MAIL", line 594
ORA-06512: at line 2