Do you make sense of this email?

I work in cybersec and one of my staff reported this email:

1. Staff **(@mycompany.com)** sends an email to a client **(@client.com and client@hotmail.com)**

2. Staff gets an email back from **postmaster@outlook.com** to **staff@hotmail.com** (which doesnt exist) that: "Delivery has failed to these recipients or groups: wongxiansheng724@gmail.com The email address you entered couldn't be found..."

The gmail address is nowhere in the chain. I checked the logs and we didnt send an email to that gmail account.

Checked the raw …

address back client cybersec cybersecurity delivery email gmail hotmail logs outlook send staff work