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One small glitch I have noticed is that when the client requests a list of messages sorted by date, the timezone is ignored.

It's not ignored, I just recently "fixed" it. Now it's properly fixed :)

Heheh.. Yes, I confirm that. (:

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It seems Dovecot is using the ,W= Maildir tag for some type of size instead of ,S= but I haven't traced far enough to work out what exactly W/virtual_size means, am I right in guessing that it might be 'wire size' i.e. message-size-with-CRLF-line-endings?

Yes. S means the actual size of the file, W is the size with CRs.

Thanks, that's good.

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And, one final question: for a dual-stack machine to listen on both IPv6 and IPv4 sockets, is the currently preferred option simply to start two instances of Dovecot with their own dovecot.conf containing different *_listen parameters?

Currently, yes. I'll probably add support for multiple listening sockets per server some day.

There is no problem to have two instances running, though it might be more admin-friendly to allow the listen addresses to be supplied on the command-line, since it would save maintaining a second config file (and I imagine this would be easier for you than changing the socket code), but this is quite a low priority I think.