Decrypting Outlook-Mail

When I move a mail from OUTLOOK to Devonthink, the mail appears in the inbox. But when I try to open it this line occurs:
“Die folgende Nachricht konnte nicht vollständig entschlüsselt werden. Nachfolgend der beste Versuch, ein lesbares Dokument aus ihr zu erstellen:”
It means
“The following message could not be completely decrypted. Below is the best attempt to create a readable document from it:”
So I must open the mail with “MAIL” and have to store it as PDF, then I have to move the PDF file to Devonthink and it will be readable. This is very labourious.
Is there a better way to make OUTLOOK-Mails be readable in Devonthink?

Which version/edition of DEVONthink do you use? In case of DEVONthink Pro/Server 3 you could try to use the alternate email view, does this help? If not - how are the emails actually encrypted?

I use Devonthink Pro V 3.5.

“Alternate email view” could help but where do I find it?

See e.g. Preferences > Email or View > Document Display

in Preferences “Document Display” does not exist. Neither “View”.

In the preferences there’s an option to enable the alternate email view, via the View > Document Display menu it’s possible to switch between both displays on the fly. This is also possible via the navigation bar. Please note that this requires at least the Pro edition.

Oh yes!!!
“Alternate view” did the trick!
Thank you very much for your advice.