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6.4.13.6 Group parameters

nnmaildir uses several group parameters. It's safe to ignore all this; the default behavior for nnmaildir is the same as the default behavior for other mail back ends: articles are deleted after one week, etc. Except for the expiry parameters, all this functionality is unique to nnmaildir, so you can ignore it if you're just trying to duplicate the behavior you already have with another back end.

If the value of any of these parameters is a vector, the first element is evaluated as a Lisp form and the result is used, rather than the original value. If the value is not a vector, the value itself is evaluated as a Lisp form. (This is why these parameters use names different from those of other, similar parameters supported by other back ends: they have different, though similar, meanings.) (For numbers, strings, nil, and t, you can ignore the eval business again; for other values, remember to use an extra quote and wrap the value in a vector when appropriate.)

expire-age
An integer specifying the minimum age, in seconds, of an article before it will be expired, or the symbol never to specify that articles should never be expired. If this parameter is not set, nnmaildir falls back to the usual nnmail-expiry-wait(-function) variables (the expiry-wait group parameter overrides nnmail-expiry-wait and makes nnmail-expiry-wait-function ineffective). If you wanted a value of 3 days, you could use something like [(* 3 24 60 60)]; nnmaildir will evaluate the form and use the result. An article's age is measured starting from the article file's modification time. Normally, this is the same as the article's delivery time, but editing an article makes it younger. Moving an article (other than via expiry) may also make an article younger.
expire-group
If this is set to a string such as a full Gnus group name, like
          "backend+server.address.string:group.name"

and if it is not the name of the same group that the parameter belongs to, then articles will be moved to the specified group during expiry before being deleted. If this is set to an nnmaildir group, the article will be just as old in the destination group as it was in the source group. So be careful with expire-age in the destination group. If this is set to the name of the same group that the parameter belongs to, then the article is not expired at all. If you use the vector form, the first element is evaluated once for each article. So that form can refer to nnmaildir-article-file-name, etc., to decide where to put the article. Even if this parameter is not set, nnmaildir does not fall back to the expiry-target group parameter or the nnmail-expiry-target variable.

read-only
If this is set to t, nnmaildir will treat the articles in this maildir as read-only. This means: articles are not renamed from new/ into cur/; articles are only found in new/, not cur/; articles are never deleted; articles cannot be edited. new/ is expected to be a symlink to the new/ directory of another maildir—e.g., a system-wide mailbox containing a mailing list of common interest. Everything in the maildir outside new/ is not treated as read-only, so for a shared mailbox, you do still need to set up your own maildir (or have write permission to the shared mailbox); your maildir just won't contain extra copies of the articles.
directory-files
A function with the same interface as directory-files. It is used to scan the directories in the maildir corresponding to this group to find articles. The default is the function specified by the server's directory-files parameter.
distrust-Lines:
If non-nil, nnmaildir will always count the lines of an article, rather than use the Lines: header field. If nil, the header field will be used if present.
always-marks
A list of mark symbols, such as ['(read expire)]. Whenever Gnus asks nnmaildir for article marks, nnmaildir will say that all articles have these marks, regardless of whether the marks stored in the filesystem say so. This is a proof-of-concept feature that will probably be removed eventually; it ought to be done in Gnus proper, or abandoned if it's not worthwhile.
never-marks
A list of mark symbols, such as ['(tick expire)]. Whenever Gnus asks nnmaildir for article marks, nnmaildir will say that no articles have these marks, regardless of whether the marks stored in the filesystem say so. never-marks overrides always-marks. This is a proof-of-concept feature that will probably be removed eventually; it ought to be done in Gnus proper, or abandoned if it's not worthwhile.
nov-cache-size
An integer specifying the size of the NOV memory cache. To speed things up, nnmaildir keeps NOV data in memory for a limited number of articles in each group. (This is probably not worthwhile, and will probably be removed in the future.) This parameter's value is noticed only the first time a group is seen after the server is opened—i.e., when you first start Gnus, typically. The NOV cache is never resized until the server is closed and reopened. The default is an estimate of the number of articles that would be displayed in the summary buffer: a count of articles that are either marked with tick or not marked with read, plus a little extra.