Imap_fetchstructure can't return values when a script includes a cycle ,for example 'while' getting parameters from mailbox using imap_fetchstructure.
example:

   while($i<300)
       ...
    $partinfo=imap_fetchstructure($mailstream,$i);
      ...
    When $i  is large than certain number such as 200 but variable by different users  and other situation,$partinfo return null array.
 

I hope that you will do me a favour!