After upgrading to D10.2 I get errors from Configuration Inspector → module, saying, that ""[site:mail]" is not a valid email address." for fields like "default_to_mail". This happens, because in issue #3364506 ✨ Add optional validation constraint support to ConfigFormBase Fixed constraint has been added.
Install D10.2, config_inspector and webform modules, create webform with email handler or save settings and in email fields, point token values in email fields. Then check configuration with config_inspector module.
I'm not sure about the best way, but as a starting point I suggest changing type from "email" to "string". It looks more appropriate, than changing core email validator, because it's our decision to support tokens as values for configuration.
Review patch, give feedback, fix patch.
Issue #3409978: Configuration validation fails for email fields with...
Automatically closed - issue fixed for 2 weeks with no activity.