If we create an Issue by mail and the project have a Post Function to add a specific person to be the watcher, this will be ignored by JIRA since it's considering the user as an anonymous user.
Also, the user who is creating the issue have the Manage Watchers permission. If he creates the issue by the UI, it works properly.

Workaround:

• Set the Manage Watchers permission to the "Anyone" group, but this is not recommended since it will give anyone the rights to manage watchers.