gnus: Article Identification

 
 6.4.13.7 Article identification
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 Articles are stored in the ‘cur/’ subdirectory of each maildir.  Each
 article file is named like ‘uniq:info’, where ‘uniq’ contains no colons.
 ‘nnmaildir’ ignores, but preserves, the ‘:info’ part.  (Other maildir
 readers typically use this part of the filename to store marks.)  The
 ‘uniq’ part uniquely identifies the article, and is used in various
 places in the ‘.nnmaildir/’ subdirectory of the maildir to store
 information about the corresponding article.  The full pathname of an
 article is available in the variable ‘nnmaildir-article-file-name’ after
 you request the article in the summary buffer.