your code must be like following.
<% String fontName="Times New Roman";
String htmlString = "<font face="+fontName+"></font>"; // incorrect without single or double quotes %>
Browser can understand 'fontName' variable without quotes but outlook can't understand it.
<% String fontName="Times New Roman";
String htmlString = "<font face=\""+fontName+"\"></font>"; //double quotes %>
OR
<% String fontName="Times New Roman";
String htmlString = "<font face='"+fontName+"'></font>"; //single quotes %>