kmail: Emails signed with Outlook certificates take very long to display

Is there any way to shorten the time to display emails signed by Outlook certificates? It takes two to three seconds to display the email and kmail seems to block during that time. I really do not care for the signature and would be happy to tell kmail to “do not check at all” because I think it tries to do some online or maybe even offline checks of the signature cert. And why does akonadi not save or at least cache the result. If the signature was valid once, why check it every time the email is viewed? But that is a different problem, I guess.

Digging into this, I think it may be related to the email address format the sender uses. Email comes from a large corporation and the format is set by their Exchange/AD server to something like this: “Doe, J., Department” <>". Checking output of kontact in a terminal, I see a bunch of these:

Input: "Doe, J., Department <>"
Error: "The email address you have entered is not valid because it contains an unexpected comma."
"Tag 'a' is not defined in message {<__kuit_internal_top__>Signiert von <a href=''>...}."

It eventually stops repeating this and then the message is displayed in kmail. So I guess, the problem is not with verifying the signature or cert but with generating the HTML to display.