Now a days lots of spam email are comming in Inbox to stop them you have to analysis your email header address and get the IP address. Enter this IP address into trace ip website section to find out the location. If you think the location is fine then you relax otherwise put this IP into spam list. Also block this IP address on your website using .htaccess file on LAMP Server.
Even when you get inquiry or online quotation from website then put the code to get the IP address of client so that you trace it easily and guess about the inquiry is fine or a spam.
Below are some of website which give you the details about the IP details: