Make the first letter title case, not all of them.

config/notifiers.go

@@ -128,7 +128,7 @@ func (c *EmailConfig) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	// Header names are case-insensitive, check for collisions.
 	normalizedHeaders := map[string]string{}
 	for h, v := range c.Headers {
-		normalized := strings.ToTitle(h)
+		normalized := strings.Title(h)
 		if _, ok := normalizedHeaders[normalized]; ok {
 			return fmt.Errorf("duplicate header %q in email config", normalized)
 		}