These options specify whether or not a channel supports the SMTP protocol and what type of SMTP line terminator IMTA expects to see as part of that protocol. nosmtp means that the channel doesn't support SMTP; all the rest of these keywords imply SMTP support.
The selection of whether or not to use the SMTP protocol is implicit for most channels; the correct protocol is chosen by the use of the appropriate channel program or programs. Some gateway systems use the Simple Mail Transfer Protocol (SMTP) described in RFC 821 as a message envelope, while others may not use an envelope format. The result is that all envelope information is derived from the RFC 822 message header, which is present in all cases. The smtp keyword is used to tell the channel master programs to put a batch SMTP header on the message. The nosmtp keyword inhibits the generation of the batch SMTP header. nosmtp is the default.
The keyword smtp is mandatory for all SMTP channels. The keywords smtp_cr, smtp_crlf, and smtp_lf may be used on SMTP channels to specify what character sequences to accept as line terminators. smtp_crlf means that lines must be terminated with a carriage return (CR) line feed (LF) sequence. smtp_lf or smtp means that an LF without a preceding CR is accepted. Finally, smtp_cr means that a CR is accepted without a following LF. It is normal to use CRLF sequences as the SMTP line terminator, and this is what IMTA always generates; this option affects only the handling of incoming material.