Debugging with Oracle's utl_smtp

set serveroutput on

declare
      lConnection UTL_SMTP.CONNECTION;
begin
      lConnection := UTL_SMTP.OPEN_CONNECTION(your_smtp_server);
      DBMS_OUTPUT.PUT_LINE('Opened ok');

      UTL_SMTP.HELO(lConnection, your_client_machine_name);
      DBMS_OUTPUT.PUT_LINE('HELO ok');

      UTL_SMTP.MAIL(lConnection, your_email_address);
      UTL_SMTP.RCPT(lConnection, your_email_address);
      DBMS_OUTPUT.PUT_LINE('Addressing ok');
end;
/

-- ****** Object: Stored Procedure TRAVELADMIN_DEV.HTML_EMAIL Script Date: 22/08/2008 12:41:02 ******
CREATE PROCEDURE "HTML_EMAIL"  (
    p_to            in varchar2,
    p_cc            in varchar2,
    p_from          in varchar2,
    p_subject       in varchar2,
    p_text          in varchar2 default null,
    p_html          in varchar2 default null
    )
is
    l_boundary      varchar2(255) default 'a1b2c3d4e3f2g1';
    l_connection    utl_smtp.connection;
    l_body_html     clob := empty_clob;  --This LOB will be the email message
    l_offset        number;
    l_ammount       number;
    l_temp          varchar2(32767) default null;
    p_smtp_hostname varchar2(30):= 'rockies';
    p_smtp_portnum  varchar2(2) := '25';
begin
    l_connection := utl_smtp.open_connection( p_smtp_hostname, p_smtp_portnum );
    utl_smtp.helo( l_connection, p_smtp_hostname );
    utl_smtp.mail( l_connection, p_from );
    utl_smtp.rcpt( l_connection, p_to );
    l_temp := l_temp || 'MIME-Version: 1.0' ||  chr(13) || chr(10);
    l_temp := l_temp || 'To: ' || p_to || chr(13) || chr(10);
    IF ((p_cc <> NULL) OR (LENGTH(p_cc) > 0)) THEN
      l_temp := l_temp || 'Cc: ' || p_cc || chr(13) || chr(10);
      utl_smtp.rcpt( l_connection, p_cc );
    END IF;
    l_temp := l_temp || 'From: ' || p_from || chr(13) || chr(10);
    l_temp := l_temp || 'Subject: ' || p_subject || chr(13) || chr(10);
    l_temp := l_temp || 'Reply-To: ' || p_from ||  chr(13) || chr(10);
    l_temp := l_temp || 'Content-Type: multipart/alternative; boundary=' ||
                         chr(34) || l_boundary ||  chr(34) || chr(13) ||
                         chr(10);
    ----------------------------------------------------
    -- Write the headers
    dbms_lob.createtemporary( l_body_html, false, 10 );
    dbms_lob.write(l_body_html,length(l_temp),1,l_temp);
    ----------------------------------------------------
    -- Write the text boundary
    l_offset := dbms_lob.getlength(l_body_html) + 1;
    l_temp   := '--' || l_boundary || chr(13)||chr(10);
    l_temp   := l_temp || 'content-type: text/plain; charset=us-ascii' ||
                  chr(13) || chr(10) || chr(13) || chr(10);
    dbms_lob.write(l_body_html,length(l_temp),l_offset,l_temp);
    ----------------------------------------------------
    -- Write the plain text portion of the email
    l_offset := dbms_lob.getlength(l_body_html) + 1;
    dbms_lob.write(l_body_html,length(p_text),l_offset,p_text);
    ----------------------------------------------------
    -- Write the HTML boundary
    l_temp   := chr(13)||chr(10)||chr(13)||chr(10)||'--' || l_boundary ||
                    chr(13) || chr(10);
    l_temp   := l_temp || 'content-type: text/html;' ||
                   chr(13) || chr(10) || chr(13) || chr(10);
    l_offset := dbms_lob.getlength(l_body_html) + 1;
    dbms_lob.write(l_body_html,length(l_temp),l_offset,l_temp);
    ----------------------------------------------------
    -- Write the HTML portion of the message
    l_offset := dbms_lob.getlength(l_body_html) + 1;
    dbms_lob.write(l_body_html,length(p_html),l_offset,p_html);
    ----------------------------------------------------
    -- Write the final html boundary
    l_temp   := chr(13) || chr(10) || '--' ||  l_boundary || '--' || chr(13);
    l_offset := dbms_lob.getlength(l_body_html) + 1;
    dbms_lob.write(l_body_html,length(l_temp),l_offset,l_temp);
    ----------------------------------------------------
    -- Send the email in 1900 byte chunks to UTL_SMTP
    l_offset  := 1;
    l_ammount := 1900;
    utl_smtp.open_data(l_connection);
    while l_offset < dbms_lob.getlength(l_body_html) loop
        utl_smtp.write_data(l_connection,
                            dbms_lob.substr(l_body_html,l_ammount,l_offset));
        l_offset  := l_offset + l_ammount ;
        l_ammount := least(1900,dbms_lob.getlength(l_body_html) - l_ammount);
    end loop;
    utl_smtp.close_data(l_connection);
    utl_smtp.quit( l_connection );
    dbms_lob.freetemporary(l_body_html);
end;

An error has occurred:
ORA-29278: SMTP transient error: 421 service not available
ORA-06512: at 'SYS.UTL_SMTP', line 17
ORA-06512: at 'SYS.UTL_SMTP', line 96
ORA-06512: at 'SYS.UTL_SMTP', line 138
ORA-06512: at 'TRAVELADMIN_DEV.HTML_EMAIL', line 20
ORA-06512: at 'TRAVELADMIN_DEV.SEND_MAIL', line 421
ORA-06512: at line 1