Note: I can’t find who to credit with this, I believe it was the IBM Notes forum but I never took a URL.
When IBM Notes 8 is installed mailto links will still launch the Standard Client. For machines which you want to launch the Standard Client the registry needs to be amended
Open the registry editor.
Navigate to [HKEY_LOCAL_MACHINE\SOFTWARE\Clients\Mail\Lotus Notes\Protocols\mailto\shell\open\command]
The (Default) value is C:\\Program Files\\IBM\\Lotus\\Notes\\notes.exe -defini %1 for 32 Bit Machines and C:\\Program Files (x86)\\IBM\\Lotus\\Notes\\notes.exe -defini %1 for 64 bit machines
Right-click (Default) and choose “Modify”
After notes.exe add ” -sa” (-sa makes notes.exe launch in the Basic Configuration client) so it looks like C:\\Program Files (x86)\\IBM\\Lotus\\Notes\\notes.exe -sa -defini %1

This can of course be done via a registry file (The registry file will remove the default key and then recreate it, but as with any registry file it should be tested first)

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Clients\Mail\Lotus Notes\Protocols\mailto\shell\open\command]
@=-
[HKEY_LOCAL_MACHINE\SOFTWARE\Clients\Mail\Lotus Notes\Protocols\mailto\shell\open\command]
@="\"C:\\PROGRA~2\\IBM\\Lotus\\Notes\\notes.exe\ -sa -defini %1\""

A further method can be done as described on (This link has stopped working before June 2017)
A further method from 8.0.2 in notes.ini is described on http://www.ns-tech.com/blog/geldred.nsf/plinks/geld-7shkqy and I would recommend this method if you change the notes.ini file on affected machines.