Most people have lots of old mail stored in various file formats. If you have set up Gnus to read mail using one of the spiffy Gnus mail back ends, you'll probably wish to have that old mail incorporated into your mail groups.
Doing so can be quite easy.
To take an example: You're reading mail using
(see Mail Spool), and have set
nnmail-split-methods to a
satisfactory value (see Splitting Mail). You have an old Unix mbox
file filled with important, but old, mail. You want to move it into
nndocgroup from the mbox file (see Foreign Groups).
All the mail messages in the mbox file will now also be spread out over
nnml groups. Try entering them and check whether things
have gone without a glitch. If things look ok, you may consider
deleting the mbox file, but I wouldn't do that unless I was absolutely
sure that all the mail has ended up where it should be.
Respooling is also a handy thing to do if you're switching from one mail back end to another. Just respool all the mail in the old mail groups using the new mail back end.