Some of our users like to send messages using BCC(blind carbon copy), thus, though the receiver got email, but he can't know other receivers. As postmaster, sometimes I'm asked to lookup information for these messages, but there is no "To:" list the message raw text.
For this, I use the system filter to log all recipients in a hidden field (in message header). Below is the filter:
# Exim filter
# Hold all recipients to prevent disclose rcpts from bcc
headers remove "X-Rrecipients"
headers add "X-Rrecipients: $recipients"
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