Return_envelope MTA option
Notification message MTA options:
return_envelope MTA option takes a bitmask value.
Bit 0 (value = 1) controls whether or not return notifications generated by the MTA are written with a blank envelope address vs. with the address of the local postmaster. Setting the bit forces the use of the local postmaster address, while clearing the bit forces the use of a blank address. Note that the use of a blank address is mandated by RFC 1123. However, some systems do not handle blank envelope From addresses properly and may require the use of this option.
Bit 1 (value = 2) controls whether or not the MTA replaces all blank envelope addresses with the address of the local postmaster. Again, this is used to accomodate incompliant systems that don't conform to RFC 821, RFC 822, or RFC 1123.
Bit 2 (value = 4) controls whether or not the MTA checks that any (non-empty) envelope From address matches (rewrites to) an MTA channel.
Setting bit 3 (value = 8) is the global (for all channels) equivalent of setting the
mailfromdnsverify channel option: it controls whether or not the MTA checks that the domain in the envelope From address resolves in the DNS. That is, setting the bit causes the MTA to require that a DNS entry can be found corresponding to the domain in the envelope From address; but the type of DNS entry does not matter.
Setting bit 4 (value = 16) causes the MTA to enforce that if the envelope From address claims a local domain name, the envelope From address must correspond to a user address (user alias).
New in 8.0, bit 6 (value = 64) modifies the effect of setting bit 3 (value = 8) on domain validity checks. With both these bits set, if the domain in the MAIL FROM address corresponds to a null MX domain, that address will be rejected as invalid. That is, setting bit 6 causes the bit 3 domain check to also implement support for draft-delany-nullmx-01.txt.
Note also that the
returnenvelope channel option can be used to impose these sorts of control on a per-channel basis.