COMMAND_SYS.Mail syntax

Hello All , 

I try to add a list of email to the to_list , here is my script : 

dest_mail_ :=  'a@gmail.com' , 'b@gmail.com' , 'b@gmail.com' ; 

COMMAND_SYS.Mail(sender_      => 'IFS Applications!',
                 from_        => '&USER_EMAIL',
                 to_list_     =>dest_mail_ ,
                 cc_list_     => '',
                 bcc_list_    => '',
                 subject_     => 'text',
                 text_        => 'Text',

                  attach_      => '',
                  rowkey_      => '',
                  mail_sender_ => 'MAIL_SENDER1');

It seems that my syntax in dest_mail_ variable is wrong , I’ve tried the semi colon , the parentheses 

Would appreciate your help .

Thank you,

Hanane