I found out that once an encrypted mail was read by OpenPGP-plugin, this mail cannot be read with default outlook SMIME. Even if I deactive the GPG-addin or I deinstall gnupg, it is not possible to read this mail in Outlooks default SMIME.
It is only possible to add a new outlook-Profile with the email-address and then the email can be read.
GpgOL needs to change an S/MIME mail in a certain way so that Outlook does not handle it internally.
We have code in GpgOL to revert this change. If you had S/MIME enabled, and then disable S/MIME in the GpgOL options (but leave GpgOL in general enabled) you have to view each “change” mail once. Once it’s parsed by GpgOL and S/MIME is disabled GpgOL will revert the change it made to the mail so that Outlook will be used for any future “views” of that mail.