After add product: SMTP server response: 503 5.5.2 Need Rcpt command
 

I've just added a few products into couple of categories but straight after adding a product to a specific category I get the following error message:

Warning: mail(): SMTP server response: 503 5.5.2 Need Rcpt command. in D:\Inetpub\wwwroot\reviews\image-inc.php on line 642

What is the problem and what's causing it? Has a user with a dodgy email address subscribed to email alerts on new products on this particular category?

Have you asked your host for more information? Any info from server logs. Reason I ask is we use a simple PHP mail string which is just like this in bold and I see nothing wrong with the code here. Maybe someone has an invalid email but without more info from the server error logs you would have a hard time having to view in sql records all the users and checking emails.

You can also clear the notify table to remove subscriptions.