In case you haven't heard, Google (provider of 30% of all email accounts) and Yahoo (2.5%) are rolling out new email authentication requirements for all email senders, effective today, to improve deliverability and prevent spam. 

These changes will affect anyone who sends email messages to personal Gmail accounts and particularly anyone who sends 5,000 messages or more to personal Gmail accounts within a 24-hour period.

The most informative advice I've seen on these changes comes from email expert Krystin Ruschman from Email2Inbox and is summarised in her recent Facebook Post (stay tuned for more).