Even the author of this blog post gets it wrong. Technically speaking, there is NO need to have a dot in the domain name. These are valid email addresses:

  user@ua  (.ua = Ukraine)
  user@km  (.km = Comoros)
  user@as  (.as = American Samoa)
  (and many more)

Because these ccTLDs have MX or A records at the top level, pointing to real MTAs. (RFCs say you should not have MX records at the top level, but many ccTLDs do it.)