All of a sudden all email accounts of my WP7 (Windows Phone 7) were no longer able to synchronize. They all mentioned “Synchronizing”, but never succeeded in syncing the mailbox.
I decided to delete the accounts using the standard method (Settings > Email & Accounts > Tap and hold on the email account you want to delete> Two options will appear “Sync” and “Delete” > Choose “Delete”), but this was not possible, as long as they were syncing.
The simplest solution was to stop all connections (3G and wifi), then I could delete the accounts. Then I enabled wifi again and added the email accounts again; now syncing was back to normal again.