I have Postfix + Maildir + Dovecot + IMAP on my mailserver. I am accessing my mailbox remotely over IMAP with one or more clients (Thunderbird). Additionally, on the server I have mutt configured for local (Maildir) access.

When I am connected over IMAP with Thunderbird, and simultaneously start mutt locally on the server, Mutt seems to be confused. When I move a message in Thunderbird (over IMAP), Mutt notices something has changed and reports:

Mailbox was externally modified.  Flags may be wrong

The problem seems to be one-sided. When I manipulate a message in Mutt, Thunderbird registers the change and has no problem with it.

Thus my question:

How can I make remote IMAP client and local Mutt client acccess Maildir without any conflicts/data corruption/errors?


I am looking for a solution where the Mutt access is local. For reasons beyond this discussion, I don't want Mutt to access the Maildir over IMAP.


I don't understand why Mutt complains that mailbox was externally modified. After all, Postfix also modifies the mailbox when it delivers new emails.

  • 4
    FYI: Mutt does the same thing when its connected over IMAP and a different IMAP client modifies the mailbox.
    – derobert
    Jul 25 '14 at 21:47

Maildir store some data in the message filename. That data include message ID, sizes (with and w/o newlines) and flags. When unread message is opened, it is moved from new to cur and its name changes from




IMAP server can track that changes cause it use maildir's index file to access certain message. When mutt access messages directly from filesystem collisions are unavoidable.

  • But as @derobert says, mutt still does the same thing when using IMAP. Jul 26 '14 at 13:16
  • 1
    But you has wrote that mutt access maildir locally and IMAP access is inappropriate?
    – Kondybas
    Jul 26 '14 at 13:19

Mutt does not "complain" about mailbox externally modified, it merely informs the user about changes resulting in simultaneous access to the same mailbox. Some cached flags might be out of sync, but generally it is not a problem. You can always reopen the mailbox to reread it. A new mail is announced differently, so this message is not seen in this situation.

Similar happens also when opening the mailbox via another ssh session or network mount, it is not IMAP specific.

There is no conflict/data corruption/error. Both mbox and maildir formats are designed for simultaneous access.

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