IMAP.MRK file format

I managed to dig up some information on the IMAP.MRK file, for anyone brave enough to want to modify it pragmatically. The format is actually fairly simple, one header followed by zero or more message records.

If a MSG file has no corresponding record, MDaemon will update the IMAP.MRK file the next time an IMAP client or WorldClient user touches the folder, and the message will be treated as UNSEEN and UNREAD

The header is defined as follows:

struct IMAPMrkHeader
unsigned HeaderVersion;
unsigned UIDValidity;
unsigned UIDNext;
unsigned LastWriteCounter;
unsigned Filler0;
unsigned Filler1;
unsigned Filler2;
unsigned Filler3;
unsigned CRLF;

Each MSG file will have one record, which is defined as follows:

struct IMAPMrkMessage {
char Filename[MAX_IMAP_FILENAME];
unsigned char Flags;
unsigned UID;
unsigned Size;
time_t Date;

#define FLAG_SEEN 32
#define FLAG_ANSWERED 16
#define FLAG_FLAGGED 8
#define FLAG_DELETED 4
#define FLAG_DRAFT 2
#define FLAG_RECENT 1

So what do all those fields mean?

  • HeaderVersion is a tag to identify what file format the IMAP.MRK is using. The current value is 1. UIDValidity is the IMAP UID Validity for the folder. UIDNext is the UID that will be used for the next message added to the folder.
  • LastWriteCounter is a value that changes whenever something has changed in the file. This was added when IDLE support was added to the IMAP server, so that there’s a quick way to check for changes (by comparing this one value rather than doing a compare of the entire file’s contents).When changing records in the file, lock the IMAP folder, increment the LastWriteCounter, change the records, then unlock the folder.
  • UIDNext is the next UID to be assigned. To add records, lock the IMAP folder, increment the LastWriteCounter, and for each record you’re adding, use the current UIDNext value as its UID and then increment UIDNext.

So how do you lock a folder?

  • Lock an IMAP folder by creating a file “IMAP-foldername-email.lck”, where foldername is a “filename-safe” version of the folder name (” and ‘/’ characters replaced with ‘_’) and email is the owner’s email address (use “public” for public folders). If the lock file already exists, something is currently operating on the IMAP.MRK so you should wait until it is unlocked before you lock it and continue.

Hope this helps someone.

CC BY-NC-ND 4.0 IMAP.MRK file format by Dave Warren ( is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

4 Replies to “IMAP.MRK file format”

  1. very usefull thanx, but i got one question.
    How whould i read the imap.mrk ?

    In case i would like to store somewhere the information for a user, if a message is unread or read , deleted or forwarded.

    In mdaemon rules there isn’t any command for reading the imap.mrk file. So how i can open it, and then fetch the information from inside? ( I already try the notepad, but probably is somehow Db File)

    Thanx for your time.

  2. There aren’t any tools included with MDaemon (other than MDaemon and WorldClient itself) that can read IMAP.MRK.

    In fact, the point of this article is, in part, to give you the information you need to write your own such tool.

    If you just want to view a mailbox, share it from the MDaemon GUI and use IMAP or WorldClient to access it.

  3. In case i need to write a program that to read the flag information from this file, it could be difficult ? can you provide some hint ? some source code? or any available website that can help with this ?

    Thank you

  4. There’s nothing particularly difficult about it, the blog article contains the header information in C format “struct”

    One other thought: the IMAP standard has no “Forwarded” flag, so although you can get “Replied” or not, you can’t get forwarded status as this isn’t known to MDaemon.

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