I think Mart and Shane have you pointed in the right direction to solve your problem with forming the commandline with the needed quotes. I did write a few FAQs on the topic.
As for how you are trying to determine the user's mailfile by splitting @FullName, you are asking for trouble. What you should do is to locate and read their Notes.INI file and grab "MailFile" from it instead.
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Give a man a fish and he will be back for more. Slap him with a fish and he will go away forever.
