nnmail-keep-last-article

nnmail-keep-last-article is a variable defined in `nnmail.el'.
Its value is

nil

Documentation:
If non-nil, nnmail will never delete/move a group's last article.
It can be marked expirable, so it will be deleted when it is no longer last.

You may need to set this variable if other programs are putting
new mail into folder numbers that Gnus has marked as expired.

You can customize this variable.