Exchange's antispam feature interrupted message receipt mid-transfer. This typically means Exchange detected the sending IP (the POPcon server) as suspicious.
Fix
- Add the POPcon server's IP address to the IP Allow List in Exchange antispam settings.
- In Exchange Management Console, go to Server Configuration > Hub Transport > Anti-Spam.
- Select IP Allow List and add the POPcon server's IP.
Alternatively, disable individual antispam agents one by one to identify which one is causing the abort.