Hello Timo,
And there is actually some (any!) way this could be avoided?... One server dies, another continues sending the mail?
I have had some thoughts about transferring idling Dovecot connections between processes / servers so that clients wouldn't notice it, but I haven't even thought about moving active (long-running) connections.
Here it is to be researched if this is specified in the IMAP standard (if there any RFC that mentions this?), or if we propose a new RFC with such an extension.
Until there is an RFC, even if you implement such a feature, there will be no clients out there that will support it.
A good start, if there is no RFC, is the http protocol, that has implemented the resume option. Like this you could even support parallel download from couple of imap servers that are synchronized, getting from each a small chunk (BitTorrent like with the seeds list being set to only the servers).
Best regards, Andrei