Here is a Lotus TechNote I came across. I didn't have time to test it for your scenario (Right-click Filename and SendTo), but it may still be of some help.
RonP
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Title: How to Launch the Notes 5.x Client from Microsoft Office Applications
Product Area: Notes
Product: Notes Client 5.x
Topic: Messaging & Mail \\ Workstation Mail (Mailer) \\ MAPI Service Providers
Number: 177998
Date: 03/23/2001
Problem:
When performing a File, SendTo, Mail Recipient in Microsoft applications, such as Word, how can it be enabled to launch the Notes client and create a Notes mail message?
Solution:
With Office 97, this functionality can be performed by enabling the option "Enable MS Office 97 SendTo to Notes" under File, Preferences, User Preferences, Additional options. This adds a Registry setting to the Office 97 Registry. Once set, performing a SendTo from the MS application will call Notes' NMAILMAN.DLL. This is a fake "MAPI" DLL that wrappers Notes OLE calls, bypassing the MAPI profile chooser code.
With Office 2000, simply open the Internet Options from the Control Panel, select the Programs tab, and select Lotus Notes under Mail. This sets the default setting for the mail registry key (under HKEY_LOCAL_MACHINE\software\clients\mail) to "Lotus Notes".
Supporting Information:
The NMAILMAN.DLL is supported ONLY in an R5 Notes Client environment. If using 4.6x, a MAPI Profile must be set up to enable access to the Domino messaging system via MAPI. The Lotus Notes client is not MAPI-enabled.
Related Documents:
"Enable MS Office 97 SendTo Notes" User Preference Not Available When Office 2000 Is Installed
Document #: 173229
Notes MAPI Service Provider Troubleshooting Guide and Known Issues
Document #: 152656
(C) Copyright 2001 International Business Machines Corporation. All rights reserved.