My client has seen an article about Nokia integrating with microsoft exchange to sync his emails between the office PC, his laptop and his Nokia E90.

Naturally enough, I'm not keen to install and setup Microsoft Windows Server with Microsoft Exchange and I'd like to present him with a linux based alternative.

Mobile phones aren't really my bag, though; can anyone point me in the right direction?

Well, I can't speak to the E90, but can speak to my past experiences with a Sony Ericcson phone. Was able to use Multisync with the Evolution plug-in, and sync my calendar, contacts, etc., easily over Bluetooth. Evolution is a pretty good replacement for Exchange. While it's Linux based, I think it's commercial software, but certainly cheaper than Exchange. The Evolution client is free, and the server supports Thunderbird and other mail/calendar clients as well.

As far as the E90 goes, here's some things you might want to look at
http://www.linuxscrew.com/2007/09/12...oth-in-ubuntu/
http://ubuntuforums.org/showthread.php?t=664406
http://www.linux-solved.com/post/Par...ird-15212.html
http://betalabs.nokia.com/blog/2009/...-in-the-future

Ovi Suite looks interesting -- uses Qt -- but as far as I can make out it's Windows-only. I couldn't find any platform requirements statement but the only download is a .exe.