How to create a rule to conditionally send a notification email, based on field value?
Here is a sample of a rule (in Rules export format), which will send an eMail to the userid selected via the field field_article_reviewer (= a field I added to my "Article" content type that is defined as an entity reference to a 'user'), and this whenever content of content type "Article" is saved:

{ "rules_notify_editor_about_outstanding_review" : {
    "LABEL" : "Notify editor about outstanding review",
    "PLUGIN" : "reaction rule",
    "OWNER" : "rules",
    "REQUIRES" : [ "rules" ],
    "ON" : { "node_presave--article" : { "bundle" : "article" } },
    "IF" : [
      { "entity_has_field" : { "entity" : [ "node" ], "field" : "field_article_reviewer" } }
    ],
    "DO" : [
      { "mail" : {
          "to" : "[node:field-article-reviewer]",
          "subject" : "Article waiting your review",
          "message" : "[node:author] has submitted a new Article post waiting your review.\r\n\r\nYou can review it at: [node:edit-url].",
          "from" : "[site:current-user:mail]",
          "language" : [ "" ]
        }
      }
    ]
  }
}


If (before importing this rule in your environment) in the above exported rule you'd change the field field_article_reviewer to match with your field name of the editor, you should be pretty close to getting it to work.

PS: I'm assuming this question is about D7.