The Agent back end, nnagent, doesn’t handle expiry. Well, at
least it doesn’t handle it like other back ends. Instead, there are
special gnus-agent-expire and gnus-agent-expire-group
commands that will expire all read articles that are older than
gnus-agent-expire-days days. They can be run whenever you feel
that you’re running out of space. Neither are particularly fast or
efficient, and it’s not a particularly good idea to interrupt them (with
C-g or anything else) once you’ve started one of them.
Note that other functions might run gnus-agent-expire for you
to keep the agent synchronized with the group.
The agent parameter agent-enable-expiration may be used to
prevent expiration in selected groups.
If gnus-agent-expire-all is non-nil, the agent
expiration commands will expire all articles—unread, read, ticked
and dormant. If nil (which is the default), only read articles
are eligible for expiry, and unread, ticked and dormant articles will
be kept indefinitely.
The last (i.e., newest) article in a group will normally not be expired (due to internal book-keeping reasons).
If you find that some articles eligible for expiry are never expired,
perhaps some Gnus Agent files are corrupted. There’s are special
commands, gnus-agent-regenerate and
gnus-agent-regenerate-group, to fix possible problems.