Description:
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I am having a very similar issue, and it is very 
reproduceable. The imap_fetchstructure(), imap_headers() and 
imap_headerinfo() calls all fail -- without returning any 
value whatsoever -- for what I have narrowed down to the 
following condition: When the attachment file size is big 
(>1MB it seems), these calls hang without returning. It is 
very reproduceable, and I did a rather comprehensive set of 
tests to determine what was triggering these calls to hang, 
and this seems to be the culprit.

Reproduce code:
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	// Open an IMAP stream using the email configuration data
	$mailbox = imap_open("{".$this->data['server']."}INBOX",$this->data['user'],$this->data['password']);
	if (!$mailbox) return false;	// If the stream is unable to be opened, return false
	
	// Extract content from messages
	for ($i=1; $i<=imap_num_msg($mailbox); $i++) {	// Iterate through all of the headers

		// Retrieve the message overview
		$header = imap_headerinfo($mailbox,$i);	// Retrieve an overview of the information contained in the message
		
print_r($header);
		// Retrieve the message structure
		$structure = imap_fetchstructure($mailbox,$i);	// Retrieve the structure of the message
print_r($structure);
		}
		


Expected result:
----------------
I would expect the header and structure to be printed for each 
message in the mailbox. This does, in fact, happen, until it 
encounters a message with an attachment that is relatively big 
(>1MB more or less). At that point, I would expect it to 
continue. Instead, the functions (pick any one of the three I 
mention) simply hang, without returning any value. This is 
connecting to a POP3 server. I can easily download the 
messages to my desktop email client, repeatedly, by the way.