Any mail send via the MailActionExecuter will include validation of email addresses. Unfortunately the address validator does not consider "local" to be a valid top level domain and thus rejects the address.
The validator used by Alfresco is slightly outdated and may not be aware of any top level domains that have recently been approved / added by IANA.
At the moment you'd have to change the email addresses you use to make sending emails work.
No - there is no option that can be set to disable the email validator in that action. I had the same kind of issue reported by a customer earlier this year who was using an ancient version of Alfresco and checked this back then - the only way to disable validation would be to apply a source-level patch to Alfresco, which I do not recommend doing except for those that consider themselves Alfresco / software development experts.