C.3.1 Opening messages is slower than expected - why? ¶

mu4e is designed to be very fast, even with large amounts of mail. However, if you experience slowdowns, here are some things to consider:

• opening messages while indexing: mu4e communicates with the mu server mostly synchronously; this means that you can do only one thing at a time. The one operation that potentially does take a bit of time is indexing of mail. Indexing does happen asynchronously, but still can slow down mu enough that users may notice.

  For some strategies to reduce that time, see the next question.

• getting contact information can take some time: especially when opening mu4e the first time and you have a lot of contacts, it can take a few seconds to process those. Note that mu4e 1.3 and higher only get changed contacts in subsequent updates (after and indexing operation), so this should be less of a concern. And you can tweak what contacts you get using mu4e-compose-complete-only-personal, mu4e-compose-complete-only-after and mu4e-compose-complete-max.
• decryption / sign verification: encrypted / signed messages sometimes require network access, and this may take a while; certainly if the needed servers cannot be found. Part of this may be that influential environment variables are not set in the emacs environment.

If you still experience unexpected slowness, you can of course file a ticket, but please be sure to mention the following:

• are all messages slow or only some messages?
• if it’s only some messages, is there something specific about them?
• in addition, please a (sufficiently censored version of) a message that is slow
• is opening always slow or only sometimes? When?

C.3.2 How can I word-wrap long lines in when viewing a message? ¶

You can toggle between wrapped and non-wrapped states using w. If you want to do this automatically, invoke visual-line-mode in your mu4e-view-rendered-hook (mu4e-view-mode-hook fires too early).

C.3.3 How can I perform custom actions on messages and attachments? ¶

See Actions.

C.3.4 How can I prevent mu4e from automatically marking messages as ‘read’ when I read them? ¶

Set mu4e-view-auto-mark-as-read to nil.

C.3.5 Does mu4e support including all related messages in a thread, like Gmail does? ¶

Yes — see Including related messages.

C.3.6 There seems to be a lot of duplicate messages — how can I get rid of them? ¶

See Skipping duplicates.

C.3.7 Some messages are almost unreadable in emacs — can I view them in an external web browser? ¶

Indeed, airlines often send messages that heavily depend on html and are hard to digest inside emacs. Fortunately, there’s an action (Message view actions) defined for this. Simply add to your configuration:

(add-to-list 'mu4e-view-actions
  '("ViewInBrowser" . mu4e-action-view-in-browser) t)

Now, when viewing such a difficult message, type aV, and the message opens inside a web browser. You can influence the browser to use with browse-url-generic-program.

C.3.8 How can I read encrypted messages that I sent? ¶

Since you do not own the recipient’s key you typically cannot read those mails — so the trick is to encrypt outgoing mails with your key, too. This can be automated by adding the following snippet to your configuration (courtesy of user kpachnis):

(require 'epg-config)
(setq mml2015-use 'epg
      epg-user-id "gpg_key_id"
      mml2015-encrypt-to-self t
      mml2015-sign-with-sender t)

C.3.9 Some plain-text attachments get corrupted, how can I avoid that? ¶

mu4e (or, more precisely, its Gnus helpers) automatically convert CR-LF (“Windows-style”) line-endings to Unix line endings for the text attachments.

That is usually convenient, but can be problematic when attachments are mislabeled as text when they contain binary data: in that case, the conversion would result in invalid data.

If you run into that problem, you can advise the responsible function:

(defun mm-dcte-keep-crlf (args)
  "Don't convert crlf->lf.
Advise `mm-decode-content-transfer-encoding' to not perform the
conversion."
  (when (and (= (length args) 2)
             (memq (nth 0 args) '(base64 x-uuencode x-uue x-binhex x-yenc))
             (string-match "\\`text/" (nth 1 args)))
    ;; fake the type, so no conversion happens
    (setf (nth 1 args) "application/octet-stream"))
  args)

(advice-add 'mm-decode-content-transfer-encoding :filter-args #'mm-dcte-keep-crlf)