Had the N900 for 18months roughly now, and got it for it's browsing prowess, and have made great use of it, but never gone over my monthly allowance, except one month where I used AGTL a lot, so put that down to mapping downloading.

I've recently started using it's email client to check email every 15 minutes when connected. (I have it set to ask for permission to connect, so only active when I am browsing or forgot to turn off)

Last two months I have gone over my 500Mb allowance in two weeks.

Just what data does it need to download to check new emails? typically I may get 2 to 6 or so in say 2 or 3 hours surfing. I have probably over 1000 in my online inbox.
It's not downloading a list of headers in the inbox every check is it?

Also, is there a better client to use than the default.
Often when I get the new email notification and click it, the Inbox shows busy for ages before showing. (lots of data?)

I don't want to check email on the web based page, as that requires me to hit check mail all the time.

not sure how much email uses, but probably your app manager is to blame for much of that data usage. try this:

http://wiki.maemo.org/Customizing_Ma..._Updates_Check

but the app manager I haven't changed for ages, if ever, and it is last two months running that have used up my allowance in 2 weeks.