Some transfers restrict the use of characters with ordinal values greater than 127 (decimal). Most notably, some SMTP servers strip the high bit and thus garble messages that use characters in this eight-bit range. IMTA 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 keyword. A channel should be marked eightbit if no such restriction exists.
Some transfers, such as extended SMTP, may actually support a form of negotiation to determine if eight-bit characters can be transmitted. The eightnegotiate keyword 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 assume that the transfer is capable of handling eight-bit data. The eightstrict keyword tells IMTA to reject any messages that contain unnegotiated eight-bit data.