There is something terribly wrong with dbmail.

For some reason, it was mangling random email headers, such that the first line of the message body was being tacked onto the final line of the message header, minus one missing character on the end of the final header line.

I managed to isolate it down to the existence of at least two single quotes characters, which are not uncommon to find in the spam scan report tacked onto the header of each delivered message. dbmail does not parse it correctly when inserting it into the sqlite database - although it has never swallowed any messages whole, as far as I can tell.

I have worked around this by altering the exim4 mail server configuration to replace any single quotes int he spam report before inserting it.

What a horrible bug.