12.4.2.1 Channel Protocol Selection and Line Terminators

Keywords: smtp, nosmtp, smtp_crlf, smtp_cr, smtp_crorlf, smtp_lf

The smtp and nosmtp keywords specify whether or not a channel supports the SMTP protocol. The smtp keyword, or one of its variations, is mandatory for all SMTP channels.

The keywords smtp_crlf, smtp_cr, smtp_crorlf, and smtp_lf can be used on SMTP channels to specify the character sequences that the MTA will accept as line terminators. The keyword smtp_crlf or smtp means that lines must be terminated with a carriage return (CR) line feed (LF) sequence. The keyword smtp_lf means that an LF without a preceding CR is accepted. The keyword smtp_cr means that a CR is accepted without a following LF. Finally, smtp_crorlf means that any of CR, LF, or the standard CRLF sequence are allowed as the SMTP line terminator. These option affect only the handling of incoming material.

Because the SMTP standard requires CRLF as the line terminator, the MTA always generates the standard CRLF sequence. The various smtp keywords merely control whether the MTA will accept additional non-standard line terminators. For example, you can specify smtp_crlf if you want the MTA to accept only strictly legal SMTP messages and reject any messages with nonstandard line terminators.