I'm looking for a desktop offline e-mail client for Linux (or Haiku) which supports:

  1. A hierarchy of labels (like in GMail).
    • An e-mail can belong to multiple labels (traditionally it is filed in a single folder).
    • Labels can be nested within each other (with 400+ labels a flat list is impractical).
    • When viewing a message, it is easy to see to which labels it belongs (so manually copying a message to several folders doesn't work, as this information is lost).
    • Colour-coded labels
  2. Conversations (like in GMail).
    • Back and forth replies are shown within the same conversation thread.
    • Searching for a message finds the thread as a whole.
    • New sent or received messages bear the same labels as the rest of the conversation.
  3. Open-source, so that I can fix bugs which affect me.

Below are features I'd like but aren't strictly necessary, things I do not care about having, and a short list of software I have already tried.

Features I'd like but really are not deal breakers:

  • Scriptable (to easily add buttons to the interface for common actions).
  • IMAP or other protocol which stores everything on the server, including the list of labels present on an e-mail (therefore POP is not very nice). Note that the IMAP protocol already has a notion of labels, but few clients expose it.
  • Good search function (I'd expect at least basic full-text search through e-mails).
  • Snooze (bury an e-mail and have it pop up later). I could script this in various ways if I really want it.

Things I do not care about having:

  • Spam filter and rule-based filters (can be done server-side).
  • Hosted solution (I can set up my own server).
  • Multiple accounts (I can forward everything to a single account).
  • I prefer a desktop application, but a local web server which works if I'm offline is ok.
  • Advanced contacts management. I would expect basic auto-complete of some form, but that's it (I can export my existing contacts list, and manage it through other means).

Things I tried:

  • GMail, which I'm currently using, meets all the functional requirements, but isn't a desktop application (offline mode in Chrome lacks access to the whole database of e-mails), and isn't open-source. Also, it fails to add existing labels in a conversation to new messages (they are displayed for the whole conversation, but actually are stored on messages, which gives inconsistencies in search).
  • Thunderbird has labels, but they are not hierarchical, and the interface to use them is cumbersome (hidden in the right-click menu, IIRC). Also, the conversation mechanism is half-broken (incoming e-mails are not sent to the folder containing the rest of the conversation), and the search functionality only turns up individual messages (then one has to laboriously find the containing conversation/folder).
  • Roundcube lacks labels (I think) and conversations, it is not a desktop application.
  • Sylpheed/Claws: no nested labels (that I know of)
  • I tried several other applications a while ago (Geary, Evolution), and I believed they lacked some of my requirements, although I don't remember the specifics.
  • I suspect that the labels can be achieved with Haiku's filesystem's rich extended attributes, with one e-mail per file. It also has a powerful search feature leveraging these extended attributes. Grouping messages by conversation might be more challenging. I haven't investigated this much, though.
  • 1
    Nylas Mail looks promissing, but I haven't tested it yet to confirm it satisfies my requirements. – Suzanne Dupéron Mar 16 '17 at 18:15
  • roundcube has threads, and they can be viewed as tree, but they are not close to ideal. – Display Name Mar 18 '17 at 9:26

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