David.
1. Get rid of the known bad bots via .htaccess if you are on an Apache server.
Here are two not updated examples:
http://multifinanceit.com/htaccess.txt
http://multifinanceit.com/htaccess1.txt
2. Authenticate your most serious visitors before they submit an email form. Collect them in a table in your database if you use that.
3. Write your email on a picture for other visitors. Collect them in another table, in your database.
4. More advanced. Write different Email classes (that may inherit structure from a more or less abstract baseclass): Email1Class, Email2Class, ..., EmailnClass where all classes allocate emails to different tables. Then you may streamline your emails from different sources to different tables. You decide how fine you will make the cathegories.