Well - you can't do that. Never really could with Nokia phones, don't know about others. Well, on some Nokia symbian phones (all?) I do recall you could do it, but that meant having two different inboxes (one separate synced with the PC though PC suite), the other fetching email from the server, but these would not stay in sync so the whole thing was pretty useless IMHO.

On the N900 you can only sync with the server, checking mail the usual way, that is what most people want anyhow I'd guess. If you use IMAP they will merrily stay in sync, or to be more precise; think of it as all your mail stays on the server, the client(s) read/delete from there. No datafiles needed on your PC, no backup issues for you.