So you want to move your mail out of Mail.app and into Gnus, the great Emacs news reader. These are the quick notes I took when I did it, and could not find a description of how to do it that actually worked. Please handle with care.
Select all the emails in the mailbox, and Save as. Chose a resonable name, and select
Raw Message Source as the Format. This will create an
C-u G fto create a new
nndocgroup. The prefix is important: without it Gnus would not ask for the format of the mailbox it is subscribing to, and it would fail to read your messages properly. Select the file name of the mailbox when prompted, and
mboxas the file type.
nndocbuffer just created, and mark all the messages for processing with
M P b.
B cto copy all the marked messages to their final destination group. When asked for the group's name you can either select an existing group create a new one, like
nnml:my.newmailbox. Note that the dot in the name will make
mya directory, and a
newmailboxmailbox in it.
total-expireset for the new mailbox, visit it and type
M Cto mark all articles as read.