Well, the scenario can be like this:

1. Someone sends email to multiple users, including our SN mailbox.

SN processes email as new (in our case - creates a CALL)

2. Any recipient of 1st mail replies with SN mailbox included.

SN processes this email as reply to CALL, based on "in-reply-to" header

This is actually the source of our problems, because we don't have replies to CALLs taken care in inbound rules and as a result email is dropped by instance. The odd thing is that this problem started after our upgrade from Istanbul to London. I've never seen such email log before and docs don't say about changes in this area between these versions...

I suppose the system does a lookup in tables which contain record with that message ID. Is there a way to exclude new_call table from this lookup? That could solve our issue in a long term...