There is no typo in the e-mail address.
Actually, the problem just popped up and I am still lost as to what is the cause. This message appears even before GO submits the mail to my SMTP engine.
I have even tried running the latest -BETA version and this problem persists.
Is there a way to debug the whole process within GO?
On another note, it appears that latest GO does not work well with Chrome-beta (18.0.1025.11 beta-m). When you click the Mail option, some blank screen comes up that blocks you from seeing the "compose", "delete", "administration" and "refresh" buttons.. well, that could be Chrome's problem anyway.