When you do @rmmail on a particular message collection, the messages removed don't go away immediately, but are rather saved elsewhere. These "zombie" messages can be brought back or examined using the @UNrmmail command.
Without `list' or `expunge', @unrmm restores the zombie messages, thus undoing the effect of the most recent @rmmail command. Only the most recent @rmmail can be undone in this way; messages deleted by any previous @rmmail commands on this same collection are lost and gone forever.
The `list' option merely lists the headers of the zombie messages without actually restoring them.
The `expunge' option banishes the zombie messages forever.
Note that the message numbers used by zombie messages are held in reserve against the possibility that you might @unrmm them some day; with such messages around, new messages received will be given higher numbers than you might have expected. @renumber does an implicit @unrmm expunge.
`@unrmmail' and `@unrmmail expunge' on collections other than your own are only allowed when you have write access. Likewise, `@unrmmail list' on other collections is only possible when they are readable by you.