Eightbit, eightnegotiate, eightstrict, sevenbit Channel Options

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Eight bit SMTP capability (eightbit, eightnegotiate, eightstrict, sevenbit)

Some transports restrict the use of characters with ordinal values greater than 127 (decimal). Most notably, some SMTP servers will strip the high bit and thus garble messages that use characters in this "eight bit" range. Indeed, there have even been past cases of SMTP servers which will crash when presented with eight bit data.

The MTA provides facilities to automatically encode such messages so that troublesome eight bit characters do not appear directly in the message. This encoding can be applied to all messages on a given channel by specifying the sevenbit channel option. A channel should be marked eightbit if no such restriction exists.

Some transports such as extended SMTP may actually support a form of negotiation to determine if eight bit characters can be transmitted. The eightnegotiate channel option can be used to instruct the channel to encode messages when negotiation fails. This is the default for all channels; channels that do not support negotiation will simply assume that the transport is capable of handling eight bit data.

The eightstrict source channel option tells the MTA to reject any messages that contain unnegotiated eight bit data.