If you are using an iPhone with IOS v4 (3G, 3GS & 4), please open this URL on your iPhone and update the Apple Airsync profile.
Issue: Apple used extremely aggressive Airsync settings in IOS v4 that can cause email to not send and raised Exchange server load.
Fix: The updated profile from Apple extends the timeouts and reduces the number of connections the iPhone opens to Exchange.
How To: The easiest way is to open the URL right on the iPhone and click the link to download the new profile. Accept it. Install it. Accept that it isn’t signed. Power off the iPhone and power back on.
Personal experience: I could not send an email via my iPhone 3G with IOS 4 installed until I updated the profile with this fix. However, I would still consider it buggy and not the final fix. The profile does make it better. I still have periodic issues with email. Apple probably needs to perform a software update to completely fix the issue.
Link: http://support.apple.com/kb/TS3398 easiest way is to open this URL right on your iPhone from this email, then click the link in step #1.