Fix headers method to handle single recipient as a string

micromail/__init__.py

@@ -172,6 +172,8 @@ class MicromailClient:
             self.write(f"Subject: {headers['subject']}\r\n".encode())
 
         to = headers.get("to", self.to)
+        if isinstance(to, str):
+            to = [to]
         self.write(f"To: {', '.join(to)}\r\n".encode())
 
     def write_message(self, content):