Regarding Dynamics CRM 2016 for Outlook client issue.

Hello I am using Dynamics CRM 2016 on premise version. I have one user having issue in Tracking one specific email only. Same user is able to track other emails. When I have enabled outlook trace, I found following issue mentioned in log,

"Entity id is null or empty, skip deleting the entity 'email'".

I don't  understand exactly what it means. Does anyone had this issue in past?