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.
- What to check if I can't add or create the Microsoft account on my Lumia phone during the initial setup?
- How can I unlock my phone so I can use another network operator’s SIM card?
- My phone does not recognise my SIM card
- Can I move or save apps to my memory card on my Lumia phone?
- I have forgotten my PIN or PUK code