The SMTP specification allows for lines of text containing up to 1000 bytes. However, some transfers may impose more severe restrictions on line length. The linelength keyword provides a mechanism for limiting the maximum permissible message line length on a channel-by-channel basis. Messages queued to a given channel with lines longer than the limit specified for that channel are automatically encoded.
The various encodings available in the IMTA always result in a reduction of line length to fewer than 80 characters. The original message may be recovered after such encoding is done by applying an appropriating decoding filter.
Note - Encoding can only reduce line lengths to fewer than 80 characters. For this reason, specification of line length values less than 80 may not actually produce lines with lengths that comply with the stated restriction.