You have several options:
1) Microsoft Office v3.0 mailbox type library (as mentioned above). Nice and simple, works well in VBA, is completely useless because it does not support WS-Security. If you use SOAP without WS-Security, you are wasting time.
2) C # lib opened in Excel using COM Interop. It is relatively simple to demonstrate the assembly for use in VBA - allows you to use WSE 3.0 / WCF in VBA. The assembly must have a strong name.
3) Visual Studio. # ( CLR) . , .NET Excel ( Word, PowerPoint ..). Visual Studio Team Suite.
( HTTPS), , , MMC snapin . , , .NET .
Rowland Watkins