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
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.