The original RFC 822 specification called for two-digit years in the date fields in message headers. This was later changed to four digits by RFC 1123. However, some older mail systems cannot accommodate four-digit dates. In addition, some newer mail systems can no longer tolerate two-digit dates.
Note - Systems that cannot handle both formats are in violation of the standards.
The datefour and datetwo keywords control IMTA's processing of the year field in message header dates. datefour, the default, instructs IMTA to expand all year fields to four digits. Two digit dates with a value less than 50 have 2000 added, while values greater than 50 have 1900 added.