In the message view (but not the headers vies), mu4e supports various
commands for dealing with mailing-lists, such as subscribing and unsubscribing.
gnus-mailing-list-subscribe: subscribe to mailing-list
gnus-mailing-list-unsubscribe: unsubscribe from mailing-list
gnus-mailing-list-help: request help for mailing-list
These commands depend on the current message to contain the necessary information for these commands.