I would create this using a Custom PHP code in rules and execute the following conditional code after saving a new node trigger : 

    if( !empty(taxonomy_get_term_by_name([node:field_tags])) ) return TRUE; 
    else return FALSE;  

Then create an 'action to send an email'. 

Requires: 
-PHP Filter and Rules enabled. 
-Your content type has to have a field for your taxonomy