There is a setting called "Default user e-mail suffix" under Manage Hudson->Configure System that allows you to set a default suffix to append to the end of email addresses, for example if you set it as "@example.com" and then in your projects you set email recipient to be "foo" it used to send the email to "foo@example.com". This setting is no longer honored and now the attempt to send the email is sent to just "foo" which of course fails.

I am unsure which version this started to fail in but it has been within the last few releases. It affects the current 1.345 and also affected 1.344 for sure.