Email Server LANMailServer

A mail filter, or Milter for short, is used to process incoming emails. Such Milters are used by Sendmail and Postfix. In LANMailServer, custom Milters can be created in the form of DLLs, allowing incoming emails received via SMTP to be modified or completely discarded. Emails retrieved from external mailboxes cannot be modified.

The service "LANMailServer Milter Service" is invoked locally by LANMailServer via TCP/IP with the respective email data. The Milter Service forwards the data to the defined Milter DLLs. The Milter Service is executed in its own memory space, separate from the LANMailServer Service.

File lmsmilters.ini

Custom Milter DLLs must be added to the file lmsmilters.ini with their drive, folder, and filename in the [MilterLibs] section with unique keys. The file lmsmilters.ini is always searched for in the LANMailServer program folder.

Example

[MilterLibs]
0=C:\Milters\MilterDLLTest.dll
1=C:\Milters\MilterTestDLL.dll

Create Your Own Milter DLL

Important: The custom Milter DLL must be compiled in 32-bit if LANMailServer is used as a 32-bit application; otherwise, it must be a 64-bit DLL.

Example for a Delphi DLL in the LANMailServer program folder file MilterDLLTest.dpr.

All strings are defined with the C++ data type BStr. Strings are initialized using the Ole2 function SysAllocString and released with SysFreeString. If assigning a custom string, the memory of the passed string must be freed using SysFreeString, then reinitialized with SysAllocString. The service will handle the release afterward.

The data type DWORD corresponds to Cardinal or longlong.

The calling convention is always stdcall, not cdecl!

The custom DLL must always export the procedure MilterInformation().

The callback functions are called during the email client-to-server connection. If the response takes too long, the client will display an error message and disconnect. The callback functions should respond quickly to avoid disconnections. The Milter Service waits a maximum of 60 seconds for a response; otherwise, it always returns MilterTempfail.

Return Values of Callback Functions

MilterOK = 0x1; MilterDenied = 0x2; MilterTempfail = 0x3; 
MilterAccept = 0x1000; MilterReject = MilterAccept + 1; 
MilterContinue = MilterReject + 1 
    
    
    
    
    

Procedure to Export

procedure MilterInformation(var MilterName: BStr; 
      
MilterVersion: PDWORD); 
    
    
    
 
    

This procedure tests the validity of the DLL; the return values are not evaluated.

Pointer to BSTR MilterName returns an identifier for the Milter using the Ole2 function SysAllocString. The Milter Service releases the string using SysFreeString.

Pointer to DWORD MilterVersion can return a version.

Optional Functions to Manipulate Email Content

function callback_envfrom(const SessionHandle: DWORD; 
var mailFrom: BStr): DWORD; 
    
    
    

Executed after the SMTP command MAIL FROM:. Envelope sender is passed.

Unique SessionHandle is passed as a DWORD.

Pointer to BSTR mailFrom, the envelope mail from is passed and can be modified. To modify, release the string with SysFreeString and initialize a new string with SysAllocString.

Return Values

MilterAccept or MilterContinue => Envelope mail from is accepted, and any changes are applied

MilterDenied or MilterReject => Envelope mail from is not accepted => Email is rejected by the server

MilterTempfail => Milter error, email is accepted

function callback_envrcpt(const SessionHandle: DWORD; 
var mailRcpt: BStr): DWORD; 
    
    
    

Executed after the SMTP command RCPT TO:.

Unique SessionHandle is passed as a DWORD.

Pointer to BSTR mailRcpt, the recipient is passed and can be modified. The call may occur multiple times for CC or BCC emails. To modify, release the string with SysFreeString and initialize a new string with SysAllocString.

Return Values

MilterAccept or MilterContinue => Recipient is accepted, and any changes are applied

MilterDenied or MilterReject => Recipient is not accepted => Email is rejected by the server

MilterTempfail => Milter error, email is accepted

function callback_header(const SessionHandle: DWORD; 
var Header: BStr): DWORD; 
    
    
    

Executed after the complete receipt of the email (SMTP command DATA .).

Unique SessionHandle is passed as a DWORD.

Pointer to BSTR Header, the complete email header is passed. To modify, release the string with SysFreeString and initialize a new string with SysAllocString.

Return Values

MilterAccept or MilterContinue => Email is accepted, and any changes are applied

MilterDenied or MilterReject => Email is not accepted => Email is rejected by the server

MilterTempfail => Milter error, email is accepted

function callback_body(const SessionHandle: DWORD; 
var Body: BStr): DWORD; 
    
    
    

Executed after the complete receipt of the email (SMTP command DATA .).

Unique SessionHandle is passed as a DWORD.

Pointer to BSTR Body, the complete email body is passed. To modify, release the string with SysFreeString and initialize a new string with SysAllocString.

Return Values

MilterAccept or MilterContinue => Email is accepted, and any changes are applied

MilterDenied or MilterReject => Email is not accepted => Email is rejected by the server

MilterTempfail => Milter error, email is accepted

function callback_abort(const SessionHandle: DWORD): DWORD;

The connection was aborted/email was not accepted by the server.

Unique SessionHandle is passed as a DWORD.

function callback_close(const SessionHandle: DWORD): DWORD;

The connection from the client to the server was interrupted.

Unique SessionHandle is passed as a DWORD.