Blimey what a pain
So is it your site got hacked and used as a relay, or is it a shared host and you have some dodgy neighbours on the same server?
“is it possible for the "message receiver" to tell the difference between an email which
purportedly comes from your email address and one which really does?
”
The simplest check for the average user is the mail will claim to be from xyz bank, or your website and the visible email will look ok, this is called spoofing and is old as the hills. Unfortunately this spoofing, or phishing as it has now evolved into gets ever more sophisticated, but normally the link it tries to send you to will give the game away as you hover your mouse over it, or right click and select properties depending on your mail client. Looking at the message headers will also give you more info about the origins.
If someone was trying to spoof your mail, then the first place to call is your ISP. Spoofing is a form of identity theft, so illegal, but is little threat for the average company until you become a recognised brand name. If you ever caught a competitor doing it, then go and talk to a solicitor
