Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

function NetSend(dest, Source, Msg: string): Longint; overload;
 type
   TNetMessageBufferSendFunction = function(servername, msgname, fromname: PWideChar;
     buf: PWideChar; buflen: Cardinal): Longint;
    stdcall;
 var
   NetMessageBufferSend: TNetMessageBufferSendFunction;
   SourceWideChar: PWideChar;
   DestWideChar: PWideChar;
   MessagetextWideChar: PWideChar;
   Handle: THandle;
 begin
   Handle := LoadLibrary('NETAPI32.DLL');
   if Handle = 0 then
   begin
     Result := GetLastError;
     Exit;
   end;
     @NetMessageBufferSend := GetProcAddress(Handle, 'NetMessageBufferSend');
   if @NetMessageBufferSend = nil then
   begin
     Result := GetLastError;
     Exit;
   end;

   MessagetextWideChar := nil;
   SourceWideChar      := nil;
   DestWideChar        := nil;

   try
     GetMem(MessagetextWideChar, Length(Msg) * SizeOf(WideChar) + 1);
     GetMem(DestWideChar, 20 * SizeOf(WideChar) + 1);
     StringToWideChar(Msg, MessagetextWideChar, Length(Msg) * SizeOf(WideChar) + 1);
     StringToWideChar(Dest, DestWideChar, 20 * SizeOf(WideChar) + 1);

     if Source = '' then
       Result := NetMessageBufferSend(nil, DestWideChar, nil,
         MessagetextWideChar, Length(Msg) * SizeOf(WideChar) + 1)
     else
     begin
       GetMem(SourceWideChar, 20 * SizeOf(WideChar) + 1);
       StringToWideChar(Source, SourceWideChar, 20 * SizeOf(WideChar) + 1);
       Result := NetMessageBufferSend(nil, DestWideChar, SourceWideChar,
         MessagetextWideChar, Length(Msg) * SizeOf(WideChar) + 1);
       FreeMem(SourceWideChar);
     end;
   finally
     FreeMem(MessagetextWideChar);
     FreeLibrary(Handle);
   end;
 end;

 function NetSend(Dest, Msg: string): Longint; overload;
 begin
   Result := NetSend(Dest, '', Msg);
 end;

 function NetSend(Msg: string): Longint; overload;
 begin
   Result := NetSend('', '', Msg);
 end;

 // Example: 

procedure TForm1.Button1Click(Sender: TObject);
 const
   NERR_BASE = 2100;
   NERR_NameNotFound = NERR_BASE + 173;
   NERR_NetworkError = NERR_BASE + 36;
   NERR_Success = 0;
 var
   Res: Longint;
   sMsg: string;
 begin
   Res := NetSend('LoginName', 'Your Message...');
   case Res of
     ERROR_ACCESS_DENIED: sMsg := 'user does not have access to the requested information.';
     ERROR_INVALID_PARAMETER: sMsg := 'The specified parameter is invalid.';
     ERROR_NOT_SUPPORTED: sMsg := 'This network request is not supported.';
     NERR_NameNotFound: sMsg := 'The user name could not be found.';
     NERR_NetworkError: sMsg := 'A general failure occurred in the network hardware.';
     NERR_Success: sMsg := 'Message sent!';
   end;
   ShowMessage(sMsg);
 end;