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 the MTA’s processing of day of the week information. The keyword dayofweek, the default, instructs the MTA to retain any day of the week information and to add this information to date and time headers if it is missing.
The keyword nodayofweek instructs the MTA to remove any leading day of the week information from date and time headers. This is intended to provide compatibility with incompliant mail systems that cannot process this information properly; it should never be used for any other purpose.