Envelopetunnel Channel Option

Envelope tunneling via header fields )
The  channel option controls the transfer of envelope information to and from special header fields defined for this purpose. The use of these fields provides a means of tunneling information associated with various SMTP extensions through systems that do not support the extensions. The option&#x27;s value is a bit-encoded integer, with each bit controlling a different header field and associated piece of envelope information. Setting the bit enables tunneling; clearing it disables it.

The default value of this option is 0, meaning all tunneling is disabled. Setting the option to -1 enables all available tunneling capabilities.

At present only one bit is defined: Bit 0 (value 1) controls the use of the MT-Priority: header line as a means of tunneling the message&#x27;s MT-PRIORITY value. The syntax and use of the field are specified in RFC 6758 (Tunneling of SMTP Message Transfer Priorities).

See also:
 * Headers channel options