qmail class large attachment and cache

Hello.

This is a two part question.

Using qmail/emailmessage to send html emails with a 7MB attachment.
If the cache option is on e.g.

$attachment=array(
"Data"=>file_get_contents($attachment_url),
"Name"=>$attachment_name,
"Content-Type"=>"automatic/name",
"Disposition"=>"attachment","Cache"=>1
);

The the send:
$error=$email_message->Send();

Just seems to hang.

I do not seem to be getting anything from the return error message either:
if(strcmp($error,""))

This also seems to happen when sending bulk emails too quickly.
I do not seem to get an error returned but the process simply seems to hang?

Any suggestions as to :1 why turningon caching could cause this.
and 2: why I often do not get an error message returned? Possibly the script has been timedout by the server???

Halting with large attachments suggests that your PHP memory limit was exceeded and PHP exited during the build of the message body in the memory. To avoid this problem you need to increase memory_limit value in the php.ini file or just send smaller files.