Copying/syncing email from a live server over network

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http://serverfault.com – The scenario is as follows: Copying and then syncing from a live mail server via network(only) to another server. The mail server is live meaning that lots of files (mails) are being altered, deleted and created. I have tried rsync but it's extremely slow and after some time I get: warning: some files vanished before they could be transferred (code 24) at main.c(1040) [sender=3.0.5] Since the server is live I would prefer not to significantly increase the load on the server. Which is the best option, preferably with cons and pros of each approach. Important facts: 15 million emai (HowTos)