[k-9-mail] K9 crashes whenever I try to reply to a specific sender

[Mark Brown]

Since update to 5.600, whenever I try to reply to one specific sender, and
only when he has cc'd his secretary into his mail to me, it crashes and I
have to restart. I cannot reply to these e-mails from him, whether I reply
to all or just to him. It doesn't happen with any other mail client, or
any other e-mail sender. I haven't imported any settings, everything was
set up from scratch.

His firm's IT department say yes, they've seen this before it's a
misconfiguration on my part (but have given as yet no further details!).
Any ideas? I can probably send a redacted example, but would obviously
prefer not to in case I don't manage to redact everything in the hidden
code that I ought to!

Thanks very much,
Mark