The target principal name is incorrect.

If you get the message:
"The server you are connected to is using a security certificate that could not be verified. The target principal name is incorrect. Do you want to continue using this server?" then this is probably what is going on.

You have SSL/TLS (secure connection) enabled in your email client and you have entered your own domain name as the incoming/outgoing mail server. That is perfectly fine in itself, but when you connect to the mail server, it checks the domain name against the certificate. The domain is then mail.yourdomain.xx and the certificate is *.webawere.nl. Because these two are different, you get this message. You can safely accept it.

The easiest way to solve this is to activate an SSL certificate for the (sub)domain you entered as the mail server. You can find more information about this in this article: https://www.weboke.nl/knowledgebase/80/Gratis-Letandsharp039s-Encrypt-certificaat-instellen.html