New York wrote:
That can happen when a new customer Registers. What is the subject (template) of the message - "Welcome ..."?
No the subject of the email is arbitrary text, sometimes Cyrillic letters sometimes text like " Stairs and fences made of spyglass, wood, metal". I don't know why I'm not making this clear enough but they can actually send emails through our shop to email addresses around the world! I see the messages in the message queue.
I'm a certified exchange server admin so I'm fairly familiar with what spoofing is and what not. You can prevent spoofing with security measures with SPF, DMARC etc.
I would appreciate some valuable feedback.