Email spoofing is the practice of sending a forged email that appears to be from a legitimate email domain, but is in fact from a different sender. Email protocols don't always verify that an address in the FROM field is one which the sending system is authorized to send from. A different (but similar) email address may be listed in the REPLY-TO field.
DKIM - DomainKeys Identified Email - is a method of authenticating the domain of an email sender.
Your Gmail account can run a DKIM authentication.
Follow these steps:
1. In a gmail message near the reply arrow, click on the drop arrow and select 'Show original'.
2. The resulting message will include a DKIM field indicating whether or not the particular message has passed the DKIM authentication test.
3. In the email header look for the line, "dkim=pass header", as official verification that the email has DKIM authentication.