Restricted, unrestricted, norestricted Channel Options

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Restricted mailbox encoding (restricted, unrestricted, norestricted)

Some mail systems have great difficulty dealing with the full spectrum of addresses allowed by RFC 822. A particularly common example of this is sendmail-based mailers with incorrect configuration files. Quoted local-parts (or mailbox specifications) are a frequent source of trouble:

"freed, ned"

This is such a major source of difficulty that a methodology was laid out in RFC 1137 to work around the problem. The basic approach is to remove quoting from the address and then apply a translation that maps the characters requiring quoting into characters allowed in an atom (see RFC 822 for a definition of an atom as it is used here). For example, the preceding address would become:

The restricted channel option tells the MTA that the channel connects to mail systems that require this encoding. The MTA then encodes quoted local-parts in both header and envelope addresses as messages are written to the channel. Incoming addresses on the channel are decoded automatically. The unrestricted channel option tells the MTA that this channel wants to see quoted addresses and that any restricted encodings should be decoded.

The norestricted channel option tells the MTA not to perform RFC 1137 encoding and decoding. norestricted is the default.

IMPORTANT NOTE: The restricted and unrestricted channel options should be applied to the channels that connects to systems unable to accept quoted or restricted local-parts respectively. They should not be applied to the channels that actually generate the quoted or restricted local-parts! (It is assumed that a channel capable of generating such an address is also capable of handling such an address.)

See also: