Problem:

How can the e-mail program be determined which is installed on the Windows PC

Resolution:

Please have a look into the registry.

Step1:

Directly under \\\\HKEY_CLASSES_ROOT the entries for the e-mail protocols pop and smtp can be found. The subentry PROTO HANDLER shows a (different) CLSID for each of used protocols.

Step 2:

This CLSID references its own entry under \\\\HKEY_CLASSES_ROOT\\CLSID.

Open the subentry belonging the CLSID found in step 1.

There, on the next level, the subentry InprocServer32 shows the DLL which handles the actual  protocol for the search started in step 1.