Hello,
Looks like spam to me.
Are you running any Symantec mail security products in your environment? Are your system scans detecting any risks?
Check the security log on your SEP client.
Could you check what Email softwares are you running on the machine? Probably there is something on the machine which is trying to send SPAM emails via port 25.
Could you please follow the article below and submit the suspicious files (if any) to the Symantec Security Response Team:
Using Symantec Support Tool, how do we Collect the Suspicious Files and Submit the same to Symantec Security Response Team.
Hope that helps!!!