I think this might be because of a conflict between Filemaker and Outlook - verify that Filemaker and Outlook have the same bit version: both must be 32bit applications, or both must be 64bit. See here: Microsoft Office / Outlook for Windows compatibility with FileMaker Pro | FileMaker
Hope this helps!
Thanks for your help.
I have Windows 10 64 bits, Outlook 2016 32 bits and FileMaker 64 bits.
So have to do a workaround via smtp or might change Outlook to 64 bits.