{"id":1638,"date":"2011-01-05T23:40:03","date_gmt":"2011-01-05T21:40:03","guid":{"rendered":"http:\/\/www.limilabs.com\/blog\/?p=1638"},"modified":"2012-07-27T11:21:25","modified_gmt":"2012-07-27T09:21:25","slug":"unique-id-in-imap-protocol","status":"publish","type":"post","link":"https:\/\/www.limilabs.com\/blog\/unique-id-in-imap-protocol","title":{"rendered":"Unique ID in IMAP protocol"},"content":{"rendered":"

RFC 3501<\/a> specification says:<\/p>\n

“2.3.1.1. Unique Identifier (UID) Message Attribute<\/em><\/p>\n

A 32-bit value assigned to each message, which when used with the unique identifier validity value (see below) forms a 64-bit value that MUST NOT<\/strong> refer to any other message in the mailbox (folder) or any subsequent mailbox (folder)<\/strong> with the same name forever. Unique identifiers are assigned in a strictly ascending<\/strong> fashion”<\/em><\/p>\n

“The unique identifier of a message MUST NOT<\/strong> change during the session, and SHOULD NOT<\/strong> change between sessions.”<\/em><\/p>\n

“Any change of unique identifiers between sessions MUST<\/strong> be detectable using the UIDVALIDITY <\/strong> mechanism”<\/em><\/p>\n

“The unique identifier validity value is sent in a UIDVALIDITY response code in an OK untagged response at mailbox (folder) selection time.”<\/em><\/p>\n

Any change of unique identifiers between session causes the change of FolderStatus.UIDValidity<\/strong>:<\/p>\n

\r\nusing(Imap client = new Imap())\r\n{\r\n    client.Connect("imap.example.org");\r\n    FolderStatus folderStatus = client.Select("Inbox");\r\n    Console.WriteLine(\r\n        "Folder UIDValidity: " + folderStatus.UIDValidity);\r\n    client.Close();\r\n}\r\n<\/pre>\n
\r\nUsing client As New Imap()\r\n    client.Connect("imap.example.org")\r\n    Dim folderStatus As FolderStatus = client.Select("Inbox")\r\n    Console.WriteLine( _\r\n        "Folder UIDValidity: " + folderStatus.UIDValidity)\r\n    client.Close()\r\nEnd Using\r\n<\/pre>\n
Uniqueness<\/h3>\n
Unfortunately: “There is no assurance that the UIDVALIDITY values of two mailboxes (folders) be different, so the UIDVALIDITY in no way identifies a mailbox (folder)<\/strong>.”<\/p>\n
This means that:<\/p>\n
    \n
  •  UID of the email may be not unique on the server (2 messages in different folders may have same UID)<\/li>\n
  •  FolderStatus.UIDValidity + UID may be not unique on the server (2 folders may have same UIDValidity)<\/li>\n
  •  To identify a message across all folders you need 3 variables: UID, folder name and FolderStatus.UIDValidity. <\/li>\n<\/ul>\n
    UIDs across sessions<\/h3>\n
      \n
    • UIDs are ‘supposed’ to be stable across sessions, and never change, and always increase in value.<\/li>\n
    • You need to check the FolderStatus.UIDValidity when you select the folder. If the FolderStatus.UIDValidity number hasn’t changed, then the UIDs are still valid across sessions.<\/li>\n<\/ul>\n
      Other<\/h3>\n
        \n
      • UIDs are assigned in ascending fashion – higher the value newer the message.<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"
        RFC 3501 specification says: “2.3.1.1. Unique Identifier (UID) Message Attribute A 32-bit value assigned to each message, which when used with the unique identifier validity value (see below) forms a 64-bit value that MUST NOT refer to any other message in the mailbox (folder) or any subsequent mailbox (folder) with the same name forever. Unique […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4],"tags":[15,28,57],"class_list":["post-1638","post","type-post","status-publish","format-standard","hentry","category-mail-dll","tag-c","tag-imap","tag-vb-net"],"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/www.limilabs.com\/blog\/wp-json\/wp\/v2\/posts\/1638"}],"collection":[{"href":"https:\/\/www.limilabs.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.limilabs.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.limilabs.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.limilabs.com\/blog\/wp-json\/wp\/v2\/comments?post=1638"}],"version-history":[{"count":9,"href":"https:\/\/www.limilabs.com\/blog\/wp-json\/wp\/v2\/posts\/1638\/revisions"}],"predecessor-version":[{"id":3290,"href":"https:\/\/www.limilabs.com\/blog\/wp-json\/wp\/v2\/posts\/1638\/revisions\/3290"}],"wp:attachment":[{"href":"https:\/\/www.limilabs.com\/blog\/wp-json\/wp\/v2\/media?parent=1638"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.limilabs.com\/blog\/wp-json\/wp\/v2\/categories?post=1638"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.limilabs.com\/blog\/wp-json\/wp\/v2\/tags?post=1638"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}