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.
- How can I transfer my phone data onto my new Lumia phone?
- What is the Transfer my Data application on my Nokia Lumia with Windows Phone 8?
- What content can I copy using Transfer my Data app from an old phone to my new Nokia Lumia?
- Does Windows Phone 7.x allow me to transfer contacts using the Transfer my Data app for Nokia Lumia?
- What type of data can I transfer from my old phone to my new Lumia Windows Phone 8?