Some of email messages have field From enclosed in quotation marks.

 Some of them are duplicates, some of them appear normal (no quotation marks) one day but the next day the same message will be enclosed in quotation marks.
What is the purpose of those marks? What are the rules creating them in the first place?

It is WLM verson independent. I am using WLM 2012 but the same problem was with the older WLM versions.

/jas