Thank you for your response.
In this case, it seems neither exchange or proxmox is temporarily recording/keeping 'user unknown' emails.
Yes they were 5xx rejections:
262DC89AE: to=<xxxxxx@xxxxxx.com>,
relay=xx.xx.xx.xx[xx.xx.xx.xx]:25, delay=1221, delays=1216/0.06/0.01/5.3,
dsn=5.1.1...