mu4e-headers-action (a) lets you pick custom actions to perform
on the message at point. You can specify these actions using the variable
mu4e-headers-actions. See Actions for the details.
mu4e defines some default actions. One of those is for capturing a
message: a c ‘captures’ the current message. Next, when you’re editing
some message, you can include the previously captured message as an
attachment, using mu4e-compose-attach-captured-message. See
mu4e-actions.el in the mu4e source distribution for more example
actions.