I only maybe once a year, get one single password rejection notice on my iphone (the only place that seems to get it), a retry and it works.
Perhaps your having too many clients all try and connect at once to collect the email?
In the long run, i do agree with Chris though.. using a 3rd party one overall is BEST practice, that you can switch carriers with no worries, having to notify, etc.
I have two primary addresses. A gmail, and a Rogers one. Only reason i use the rogers one, is that its almost 20 years old :P (was originally a rogerswave email)