How can i avoid a mutating problem with a custom event that sends emails when an application message finishes?

Create a PL/SQL block that sends an email via a background job. (My example assumes you have a mail sender MAIL_SENDER_HTML. Yours is probably something like MAIL_SENDER1 if you haven’t changed it.)

DECLARE
   attr_ VARCHAR2(32767);
BEGIN
   client_sys.clear_attr(attr_);
   client_sys.add_to_attr('FROM_USER_NAME_', fnd_session_api.get_fnd_user, attr_);
   client_sys.add_to_attr('TO_USER_NAME_',   fnd_session_api.get_fnd_user, attr_);
   client_sys.add_to_attr('SUBJECT_',        'This is your subject line.', attr_);
   client_sys.add_to_attr('TEXT_',           'This is your message body.', attr_);
   client_sys.add_to_attr('MAIL_SENDER_',    'MAIL_SENDER_HTML',           attr_);
   -- client_sys.add_to_attr('ERROR_TEXT_', NULL, attr_);
   -- client_sys.add_to_attr('ATTACH_',     NULL, attr_);
   -- client_sys.add_to_attr('CC_',         NULL, attr_);
   -- client_sys.add_to_attr('TIMER_',      NULL, attr_);
   -- client_sys.add_to_attr('AUDIT_',      NULL, attr_);
   -- client_sys.add_to_attr('FROM_ALIAS_', NULL, attr_);
   -- client_sys.add_to_attr('LABEL_',      NULL, attr_);
   transaction_sys.deferred_call(
      'command_sys.mail',
      'PARAMETER',
      attr_,
      'This is your background job description.');
END;
/