FoldLines is a Boolean property used to indicate if the header value is separated into one or moe lines using the value in FoldLength. The default value for FoldLines is True as assigned in the Create constructor. 

FoldLines is used when writing a new value to Values, and forces the value in the header to be wrapped at the number of characters indicated in FoldLength. 

Set UnfoldLines to indicate the line folding is used when reading a header in Values.

TIdHeaderList.FoldLength 

TIdHeaderList.Values