It's hard to say exactly what the problem is. Being an Android fan myself, I can say I've seen the issue across multiple email providers, so while I can't say for certain, I think it's actually an issue with the way Android handles the mail account information.
I know this is a pain, but can I recommend that you delete the account from your device and then recreate it? I've found this to solve my problem after Android upgrades on no less than 5 different email provider accounts.
