### Description

When using the Watcher email action, it would be beneficial to be able to specify a list of allowed domains to which emails are allowed to be sent. This would probably be a non-dynamic setting, and allow specifying multiple domains.
x-pack/plugin/watcher/src/main/java/org/elasticsearch/xpack/watcher/notification/email/EmailService.java
);
    private static final Setting.AffixSetting<TimeValue> SETTING_SMTP_TIMEOUT = Setting.affixKeySetting(
        "xpack.notification.email.account.",
        "smtp.timeout",
        (key) -> Setting.timeSetting(key, TimeValue.timeValueMinutes(2), Property.Dynamic, Property.NodeScope)
    );
    private static final Setting.AffixSetting<TimeValue> SETTING_SMTP_CONNECTION_TIMEOUT = Setting.affixKeySetting(
        "xpack.notification.email.account.",
        "smtp.connection_timeout",
        (key) -> Setting.timeSetting(key, TimeValue.timeValueMinutes(2), Property.Dynamic, Property.NodeScope)
    );
    private static final Setting.AffixSetting<TimeValue> SETTING_SMTP_WRITE_TIMEOUT = Setting.affixKeySetting(
        "xpack.notification.email.account.",
        "smtp.write_timeout",
        (key) -> Setting.timeSetting(key, TimeValue.timeValueMinutes(2), Property.Dynamic, Property.NodeScope)
    );
    private static final Setting.AffixSetting<String> SETTING_SMTP_LOCAL_ADDRESS = Setting.affixKeySetting(
        "xpack.notification.email.account.",
        "smtp.local_address",
        (key) -> Setting.simpleString(key, Property.Dynamic, Property.NodeScope)
    );
    private static final Setting.AffixSetting<String> SETTING_SMTP_SSL_TRUST_ADDRESS = Setting.affixKeySetting(
        "xpack.notification.email.account.",
        "smtp.ssl.trust",
        (key) -> Setting.simpleString(key, Property.Dynamic, Property.NodeScope)
    );
    private static final Setting.AffixSetting<Integer> SETTING_SMTP_LOCAL_PORT = Setting.affixKeySetting(
        "xpack.notification.email.account.",
        "smtp.local_port",
        (key) -> Setting.intSetting(key, 25, Property.Dynamic, Property.NodeScope)