When using 1&1 MailXchange E-mail system, one can use sync clients for MS Outlook (Desktop and Mobile). I am using the mobile sync client and it works like a charm.

On my computers I don't use MS Outlook, but Thunderbird and Lightning instead. As Outlook, these don't provide native SyncML connectors. 1&1 doesn't offer much of a help about the SyncML used, but with a little web search and trial-and-error I was able to find a way to sync Thunderbird and Lightning with my MailXchange account.