We were informed by our spam filter provider, SonicWall that they were experiencing a "denial of service" attack (also known as DOS) last week causing a slow-down of external e-mail being delivered into TLU. This was not just a TLU problem but one that was impacting other clients as well. As of 10/7/17, the situation had stabilized and regular delivery was proceeding.
Note: This was only impacting inbound e-mail from external sources.