When I send emails with attachments they are correctly sent and I can see them in most clients but the paperclip is not showing.

Outlook shows this behaviour.

This wouldnt be a big problem if wheren't for other email clients like "bluewin.ch" where it doesn't even show the attachments when the paperclip is not available.

EML Builder code:

public static class EmlBuilder {

        private const string NewLine = "\r\n";

        /// <summary>
        /// Builds and .eml File https://en.wikipedia.org/wiki/MIME#Content-Type
        /// Opens it 
        /// And cleans up the temporary file as soon as it isn't needed anymore
        /// </summary>
        public static void BuildAndOpen(string receiver = null, string subject = null, string signature = null, params string[] attachments) {
            //header
            var version          = "MIME-VERSION: 1.0";
            var draftFlag        = $"X-Unsent: {true.ToInt()}";
            var from             = "From: <>"; 
            var xReceiver        = $"X-Receiver: {receiver}";
            var to               = $"To: {receiver}";
            var subj             = $"Subject: {subject}";

            //boundaries
            var boundary         = $"frontier";
            var contentType      = $"Content-Type: multipart/mixed; boundary={boundary}";
            var boundaryLine     = $"--{boundary}";

            //header-content
            var headerContent = string.Empty;
            headerContent      = version       NewLine; 
            headerContent      = draftFlag     NewLine;
            headerContent      = contentType   NewLine;
            headerContent      = from          NewLine;

            if (subject.IsDefined()) {
                headerContent  = subj          NewLine;
            }
            if (receiver.IsDefined()) {
                headerContent  = xReceiver     NewLine;
                headerContent  = to            NewLine;
            }

            //between header and body
            var headerBodyBoundary = string.Empty;
            headerBodyBoundary  = boundaryLine                               NewLine;
            headerBodyBoundary  = "Content-Type: text/html"   NewLine; 

            //body-content
            var bodyContent = string.Empty; 
            bodyContent     = string.Empty   NewLine; //TODO settings

            //add signature
            bodyContent  = signature;

            //all attachments after body
            var fullAttachmentContent = string.Empty;

            var attachmentBoundary      = string.Empty;
            attachmentBoundary          = boundaryLine                                NewLine;
            attachmentBoundary          = "Content-Type: application/{0}; name={1}"   NewLine;
            attachmentBoundary          = "Content-Transfer-Encoding: Base64"         NewLine;
            attachmentBoundary          = "Content-Location: attachment"              NewLine;
            attachmentBoundary          = string.Empty                                NewLine;
            var singleAttachmentContent = string.Empty;
            singleAttachmentContent     = "{0}"                                       NewLine;

            foreach (var attachment in attachments) {
                fullAttachmentContent  = NewLine.Combine(
                    string.Format(attachmentBoundary,      Path.GetExtension(attachment), Path.GetFileName(attachment)), 
                    string.Format(singleAttachmentContent, Convert.ToBase64String(File.ReadAllBytes(attachment)))
                );
            }

            //final-file
            var tmpPath       = Path.GetTempPath();
            var date          = DateTime.Now.ToString("yyyyMMddTHHmmssfff");
            var guid          = Guid.NewGuid().ToString("N"); //no-hyphens
            var fileExtension = ".eml";
            var filePath      = Path.Combine(tmpPath, "_".Combine(date, guid)   fileExtension);
            File.WriteAllText(filePath, NewLine.Combine(headerContent, headerBodyBoundary, bodyContent, fullAttachmentContent), Encoding.Default);

            var            clients            = Applications.GetRunningEmailClients();
            List<Process>  unmanagedProcesses = new List<Process>();
            clients.ForEach(c => unmanagedProcesses.AddRange(c.GetProcesses()));

            //open file
            var managedProcess = Process.Start(filePath);

            //this is a workaround for the default behaviour of email clients
            //if a client is running when opening an .eml file, then the process of starting the file
            //exits immediately and is handed over to the already running client instance
            //at this point it's impossible to tell if the file is free to clean up yet

            Task.Run(() => {
                while (!managedProcess.HasExited) { Thread.Sleep((int)TimeSpan.FromSeconds(60).TotalMilliseconds); }
                if (unmanagedProcesses.Any()) unmanagedProcesses.ForEach(Tools.WaitForHasExited);
                try {
                    File.Delete(filePath);
                }
                catch { /*leave file orphaned*/ }
            });
        }
    }

I don't even know how to define a google search for this problem since I can't avoid using very highly used words like "attachment", "email", "outlook", beside others.

I would appreciate any ideas on how to fix or debug this.

Feel free to ask if any further information is needed.

CodePudding user response：

The paperclip icon visibility in Outlook is determined by the SmartNoAttach named property (DASL name "http://schemas.microsoft.com/mapi/id/{00062008-0000-0000-C000-000000000046}/8514000B"), the property can be set using MailItem.PropertyAccessor.SetProperty - set the it using the above DASL name to false.

CodePudding user response：

I've found the problem.

The line:

attachmentBoundary  = "Content-Location: attachment"   NewLine;

Was suposed to be

attachmentBoundary  = "Content-Disposition: attachment"   NewLine;

With this change the attachments are shown correctly.