TYPO3 Core

Good catch. With #98443, the linkHandler identifier has been renamed from mail to email. Therefore, buttons.link.mail.properties.class.default in your YAML file has no effect and can be changed to buttons.link.email.properties.class.default in order to get the default link class back.

I'm unsure if this renaming happened on intention or by mistake and will try to find out, if configuration must be adapted manually or if the identifier can be changed back to mail or if a automatic migration can be added.

Ohhh yes, I overlooked the change and adjusted my configuration accordingly - works.
Many thanks for your help @Torben Hansen