Evolution Header Format

Subject: header test
From: Username <username@sendinghost.com>
To: Username <username@receivinghost.com>
Content-Type: text/plain
Date: Sat, 28 Jul 2007 11:52:35 +0200
Message-Id: <1185616355.19231.0.camel@localhost>
Mime-Version: 1.0
X-Mailer: Evolution 2.10.1 
Content-Transfer-Encoding: 7bit

Code responsible for generating Message-ID:

camel_header_msgid_generate (void)
{
        static pthread_mutex_t count_lock = PTHREAD_MUTEX_INITIALIZER;
#define COUNT_LOCK() pthread_mutex_lock (&count_lock)
#define COUNT_UNLOCK() pthread_mutex_unlock (&count_lock)
        char host[MAXHOSTNAMELEN];
        char *name;
        static int count = 0;
        char *msgid;
        int retval;
        struct addrinfo *ai = NULL, hints = { 0 };

        retval = gethostname (host, sizeof (host));
        if (retval == 0 && *host) {
                hints.ai_flags = AI_CANONNAME;
                ai = camel_getaddrinfo(host, NULL, &hints, NULL);
                if (ai && ai->ai_canonname)
                        name = ai->ai_canonname;
                else
                        name = host;
        } else
                name = "localhost.localdomain";

        COUNT_LOCK ();
        msgid = g_strdup_printf ("%d.%d.%d.camel@%s", (int) time (NULL), getpid (), count++, name);
        COUNT_UNLOCK ();

        if (ai)
                camel_freeaddrinfo(ai);

        return msgid;
}

References

evolution-data-server/camel/camel-mime-utils.c