This line, if present, is in a legal USENET date format. It specifies a suggested expiration date for the message. If not present, the local default expiration date is used. This field is intended to be used to clean up messages with a limited usefulness, or to keep important messages around for longer than usual. For example, a message announcing an upcoming seminar could have an expiration date the day after the seminar, since the message is not useful after the seminar is over. Since local hosts have local policies for expiration of news (depending on available disk space, for instance), users are discouraged from providing expiration dates for messages unless there is a natural expiration date associated with the topic. System software should almost never provide a default Expires line. Leave it out and allow local policies to be used unless there is a good reason not to.
RFC1036: [Source:"RFC-1036"] [Last Changed:December 1987] [Copyright: 1987 M. Horton, R. Adams]
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Up to RFC1036 2.2. Optional Headers