How To Force a Particular Internet Encoding by Using MAPI

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Summary

You can use the Messaging Application Programming Interface (MAPI) property PR_INETMAIL_OVERRIDE_FORMAT (0x59020003) to force the Exchange Internet Mail Service (IMS) to use a particular encoding scheme for the message body and attachments. This per-message property can be set in both Collaboration Data Objects (CDO) and Extended MAPI code.

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More information

The possible values of PR_INETMAIL_OVERRIDE_FORMAT, which correspond to the Send Options settings in Microsoft Outlook, are as follows:

Value	Outlook Setting	Message Format	Attachment Encoding
0	Unknown	Depends on default IMS settings	Depends on default IMS settings
1	MIME	MIME	Base64
2	UUEncode	Text	UUEncode
3	BINHEX	Text	BinHex

Sample Code for CDO

The following CDO Microsoft Visual Basic code sends a MIME message with two attachments, encoded with Base64:

' Reference to Microsoft CDO 1.21 library.
Private Sub SendMail_Click()
    Dim objSession As MAPI.Session
    Dim objMessage As MAPI.Message
    Dim objAttachments As MAPI.Attachments
    Dim objAttachment As MAPI.Attachment
    Dim objRecipients As MAPI.Recipients
    Dim objRecipient As MAPI.Recipient
       
    Dim Recipient As String
    Dim MsgSubject As String
    Dim EncodingFlag As Integer
    Dim bmpFile As String
    Dim txtFile As String
    Dim FileLocation As String
    
    'TODO: Modify the following to appropriate values.
    Recipient = "myaccount@hotmail.com"
    EncodingFlag = 1 'Use MIME encoding
    bmpFile = "mybmp.bmp"
    txtFile = "mytxt.txt"
    FileLocation = "c:\"
    
    MsgSubject = "Test Encoding: " & EncodingFlag
    
    On Error GoTo Err
    
    Set objSession = CreateObject("MAPI.Session")
    
    objSession.Logon showdialog:=True
    
    Set objMessage = objSession.Outbox.Messages.Add(MsgSubject, "body")
        
    Set objAttachments = objMessage.Attachments
    objAttachments.Add txtFile, 0, CdoFileData, FileLocation & txtFile
    objAttachments.Add bmpFile, 0, CdoFileData, FileLocation & bmpFile
    
    Set objRecipients = objMessage.Recipients
    Set objRecipient = objRecipients.Add
    
    objRecipient.Name = Recipient
    objRecipient.Resolve
        
    objMessage.Fields.Add &H59020003, EncodingFlag
    
    objMessage.Update
    
    objMessage.Send True
    
Err:
    If Err.Number Then
        Debug.Print Err.Number & ": " & Err.Description & "::" & Err.Source
        Err.Clear
    End If
    
    If Not objSession Is Nothing Then objSession.Logoff

End Sub

An issue in the CDO ReadFromFile method can interfere with the encoding process. If ReadFromFile is used, CdoPR_ATTACH_LONG_FILENAME needs to be set on the attachment. For more information, see the following article in the Microsoft Knowledge Base:

278358 PRB: File Attached Via CDO Has Incorrect Filename

Headers for MAPI

You can use this property from Extended MAPI code as well; the following #defines will help:

#define PR_INETMAIL_OVERRIDE_FORMAT PROP_TAG(PT_LONG,0x5902)

#define ENCODEDONTKNOW 0
#define ENCODEMIME 1
#define ENCODEUUENCODE 2
#define ENCODEBINHEX 3

Important Notes

The encoding specified in PR_INETMAIL_OVERRIDE_FORMAT take effect only if the IMS does not encode the message as Rich Text/TNEF. For information on preventing the use of transport-neutral encapsulation format (TNEF), see the following article:

149203 XFOR: Preventing Winmail.dat From Being Sent Over IMC

The information in this article applies only to mail that is sent through the Exchange Internet Mail Connector (IMC). It does not apply to mail that is sent through the Outlook Internet Mail Provider.

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