Microsoft seems to have kind of forgotten to integrate the MAPI mapping into the modern settings, but the old default app settings via control panel are gone with v1709.
So the first step has to be done via Registry:
HKEY_CURRENT_USER\SOFTWARE\Clients\Mail -> Set (Default) e.g. to "Microsoft Outlook", "eM Client" etc.
HKEY_LOCAL_MACHINE\SOFTWARE\Clients\Mail -> Same here
#Look at the subkeys here to get the right values if you are using an alternative email client
Because this does not always solve the problem, we also have to remove this file once:
“C:\Program Files (x86)\Common Files\system\MSMAPI\1031\MSMAPI32.DLL”
(Use C:\Program Files\ if you have Office x64 installed)
Afterwards, open your e-mail application. E.g. Outlook runs Fixmapi repair itself for a few seconds (no admin rights should be necessary), then it should run again without having any settings lost.
If you want to use group policy to delete the DLL on all workstations once, make sure you apply the rule only to computers which have already been upgraded to 1709.
E.g. this WMI filter works for v1709 and upwards:
SELECT * FROM Win32_OperatingSystem where BuildNumber >= 16299
Then use the group policy File preferences to delete the DLL exactly once.