The RFC 822 specification allows for a leading day of the week specification in the date fields in message headers. However, some systems cannot accommodate day of the week information. This makes some systems reluctant to include this information, even though it is quite useful information to have in the headers.
The dayofweek and nodayofweek keywords control IMTA's processing of day of the week information. dayofweek, the default, instructs IMTA to retain any day of the week information and to add this information to date/time headers if it is missing.