Imap username MTA option

BURL MTA options:  (string)
In order to perform an IMAP BURL operation, the SMTP SUBMIT server has to have the ability to log in to the IMAP server as the submit user. The  MTA option specifies the  submit user; if not set, it defaults to the setting of the    option (corresponding to the old  configutil parameter  ).

See also:
 * imap_password MTA Option
 * BURL MTA options
 * submituser Option
 * BURL_ACCESS mapping table