I need to change hosting for many domains, what should I consider? [on hold]

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http://superuser.com – I work at a small web developer company which primarily works with Joomla!. We need to switch from one hosting to another. What should I think about? Copy all sites via FTP Copy all Databases (export, create new, import) Mail... We use smartmail at the moment and are gonna use ubuntu with Plesk-panel. This is the part I'm most worried about. How should i migrate all mailaccounts? (HowTos)