diff --git a/writeto.go b/writeto.go
index 9fb6b86..ac12357 100644
--- a/writeto.go
+++ b/writeto.go
@@ -137,10 +137,11 @@ func (w *messageWriter) addFiles(files []*file, isAttachment bool) {
 		}
 
 		if !isAttachment {
-			if _, ok := f.Header["Content-ID"]; !ok {
-				f.setHeader("Content-ID", "<"+f.Name+">")
+			if _, ok := f.Header["Content-Id"]; !ok && f.Header["Content-ID"] == nil {
+				f.setHeader("Content-Id", "<"+f.Name+">")
 			}
 		}
+
 		w.writeHeaders(f.Header)
 		w.writeBody(f.CopyFunc, Base64)
 	}