There seems to be a glitch on the iPhone 3G when it comes to deleting emails. Through one of my IMAP accounts, when I try to delete an email, I consistently get the error “Unable to Move Message”. When the email account was setup on the iPhone, it was already selected to move deleted emails to the local trash folder of the phone. If you are also unable to delete emails on your iPhone, try this. Go inside:
Settings > Mail, Contacts, Calendar > Email Account > Advanced > Deleted Mailbox
Then tap on Trash – even though you may already have a check mark beside it. This should allow you to start deleting emails.
Update: Depending on how you setup your email account, you may need to click one extra step to find Advanced:
Settings > Mail, Contacts, Calendar > Email Account > Email Account (again) > Advanced > Deleted Mailbox
Then tap on Trash.