Lost messages with RPOP

We have a strange problem with our installation.
Messages are collected via RPOP from a provider. One of our partner has sent a message that we did not receive while we continued to receive other messages from him. He resend and forwarded the message several times and we have never received it.

Digging deeper we could see the message arrive to the provider, where it then disappeared due to the RPOP collection from Axigen. However, it never pops up anywhere in Axigen. Not in Spam, not in Filtered Mails. The sender domain is whitelisted and - as mentioned - his other messages come through without problems.

Even with log level set to highest the “everything” log does not show details about the messages collected during RPOP so we have no idea what is going on.

Any idea ? Thanks !

I would be so thankful if somebody could help with this !