The @unsend command was created by ballot on LambdaMOO, *B:Unsend (#77081).
Syntax: @unsend [message-sequence] from <user>
A message cannot be unsent to a player if e has read the message, it has been forwarded to eir e-mail address, or e has set an option to prevent messages being unsent to em. A message cannot be unsent to a group of players unless it can be unsent to each player in the group.
The unsender may not view a list of unread messages on a player. E may only select the message(s) to be unsent by filters such as "subject:<subject>" or "since:today".
In order to protect privacy, if any of the restrictions above, such as one of the users having already read the message, are not met, the error message will be quite generic and not tell you exactly what went wrong, but simply that messages could not be unsent.
The following mail options are available:
@mail-option +no_unsend No one may @unsend messages they sent to you
@mail-option -no_unsend People may @unsend unread messages they send to you
@mail-option @unsend=<message sequence>
sets the default message sequence that the @unsend command will operate upon. The default is "last:1".