The mailbox may have some emails with an excessively long subject line that are not supported by Windows Phone 8. Once you have removed the suspect mail item (i.e. cleaned the mailbox or move the suspect mail to a folder that phone does not sync) the error will cease.
- Are there limitations for Hey Cortana on my Lumia?
- What to check if I can't sign in or create the Microsoft account on my Lumia phone during the initial setup?
- How can I train Hey Cortana on my Lumia phone?
- How does Hey Cortana work when Battery Saver mode is on?
- Will I get “Hey Cortana” voice activation after Lumia Denim update?