The Mail Watcher Plugin does not appear on the panel when added. Even when it does appear in items in panel preferences. This means that it is not possible to access the Mail Watcher Plugin preferences.
A possible separate issue is that clicking on Add for the Mail Watcher Plugin causes an error informing you that the plugin had failed to start multiple times and asking if it should be removed. (I'm just including this for completeness of what I have found, and I suspect it is not related to the main issue.)
Arch package versions:
Just had an upgrade to Arch package versions:
and everything is woking. Thanks.