When receiving a reply to a locked topic, mail handler crashes with the following error:

  undefined local variable or method `sender_email' for #
Did you mean?  send_action
  app/models/mail_handler.rb:289:in `receive_message_reply'

That's because @receive_message_reply@ is trying to use @sender_email@ variable, which was not defined in this method.
Here is the corresponding line (source:trunk/app/models/mail_handler.rb@18198#L277):

logger&.info "MailHandler: ignoring reply from [#{sender_email}] to a locked topic"