Dropblank, nodropblank Channel Options

Strip illegal blank recipient headers
In RFC 822 messages, any To:, Resent-To:, Cc:, or Resent-Cc: header is required to contain at least one address - such a header may not have a blank value. Nevertheless, some mailers may emit such illegal headers. The  channel option, if specified on a source channel, causes the MTA to strip any such illegal blank headers from incoming messages. disables this action and is the default.

See also:
 * Headers channel options
 * Channel options