I use K9-Mail v8.2 on a Pixel 7a with Android 15 (GrapheneOS).
When I recently had a certificate error, I unfortunately had to act quickly. That’s why I permanently accepted an invalid certificate in K9 Mail.
Gear icon > IMAP mailbox > Retrieve emails > Incoming mail server > Settings Incoming mail server to accept > Next > Advanced > Accept risk and continue
How do I permanently undo this?
Did I formulate my question imprecisely? Is information missing? 
I for one am not ignoring you, I simply don’t know the answer. I never had to deal with any certificates in K-9/TBird, invalid or otherwise - sorry 
Not sure about Graphene, but regular Android:
Settings → Security and Privacy → More Security and Privacy → Encryption and Credentials → Trusted Credentials → User
If it’s not there go back two steps to “User Credentials”.
You should be able to tap the certificate and either disable or delete it.
Many thanks for your willingness to help! 
I thought I could solve this within K9, because DAVx⁵ has Reset certificate in the menu…
In the Android settings, I can’t find any trusted credentials from the user but a frightening amount from the system.
How can I find the correct credentials?
The problem occurred when I accessed my Synology NAS via IP in the LAN. And I can’t find Synology. But there are 5x SSL Corporatrion.