The descriptor throws an error when you attempt to set a value for smtpHost using jcasc plugin.

The error message is identical to this reported github issue and subsequently JENKINS-53467 for the Mailer plugin which was fixed by introducing a getSmtpHost() method. It appears the solution for this plugin may not be as trivial however; I noticed a getSmtpHost() method is already present in MailAccount.java .

SEVERE: Failed ConfigurationAsCode.init
java.lang.Error: java.lang.reflect.InvocationTargetException
at hudson.init.TaskMethodFinder.invoke(TaskMethodFinder.java:110)
at hudson.init.TaskMethodFinder$TaskImpl.run(TaskMethodFinder.java:175)
at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:296)
at jenkins.model.Jenkins$5.runTask(Jenkins.java:1068)
at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:214)
at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:117)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at hudson.init.TaskMethodFinder.invoke(TaskMethodFinder.java:104)
... 8 more
Caused by: io.jenkins.plugins.casc.ConfiguratorException: unclassified: error configuring 'unclassified' with class io.jenkins.plugins.casc.impl.configurators.GlobalConfigurationCategoryConfigurator configurator
at io.jenkins.plugins.casc.ConfigurationAsCode.invokeWith(ConfigurationAsCode.java:611)
at io.jenkins.plugins.casc.ConfigurationAsCode.checkWith(ConfigurationAsCode.java:640)
at io.jenkins.plugins.casc.ConfigurationAsCode.configureWith(ConfigurationAsCode.java:627)
at io.jenkins.plugins.casc.ConfigurationAsCode.configureWith(ConfigurationAsCode.java:539)
at io.jenkins.plugins.casc.ConfigurationAsCode.configure(ConfigurationAsCode.java:270)
at io.jenkins.plugins.casc.ConfigurationAsCode.init(ConfigurationAsCode.java:262)
... 13 more
Caused by: io.jenkins.plugins.casc.ConfiguratorException: Invalid configuration elements for type class hudson.plugins.emailext.ExtendedEmailPublisherDescriptor : smtpHost
at io.jenkins.plugins.casc.BaseConfigurator.handleUnknown(BaseConfigurator.java:355)
at io.jenkins.plugins.casc.BaseConfigurator.configure(BaseConfigurator.java:345)
at io.jenkins.plugins.casc.BaseConfigurator.check(BaseConfigurator.java:265)
at io.jenkins.plugins.casc.BaseConfigurator.configure(BaseConfigurator.java:330)
at io.jenkins.plugins.casc.BaseConfigurator.check(BaseConfigurator.java:265)
at io.jenkins.plugins.casc.ConfigurationAsCode.lambda$checkWith$8(ConfigurationAsCode.java:640)
at io.jenkins.plugins.casc.ConfigurationAsCode.invokeWith(ConfigurationAsCode.java:605)
... 18 more