Your sample HTML do not even compile...
Read about string literals (and how you can include string separator - double-quote - in string):
2.4.4.5 String literals (C#)[
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Escape Sequences[
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A possible clean version of your line:
STRBODY += "<span style=\"font-weight:bold\">TaskID:< /span>" + a[GUIDCollection.TASK_TASKID_GUID].ToString() + "<span style=\"font-weight:bold\">CategoryName:< /span>" + Category.Name.ToString() + " " + Status.Name.ToString();