I've just tracked down a long standing problem I've had with the IMP webmail 
problem and large attachments.  I've configure IMP 3.1/PEAR 1.0.1 to use SMTP 
to talk to a local mail server.  Sending a message with a 13MB attachment (19 
MB encoded) causes the memory usage to shoot up to 122MB. This seems to be due 
to the handling of the message body with regards to string copies and regular 
expression replacements.

This effectively limits the size of attachements that are useable within imp 
not to mention puts a pretty big strain on my server.


In particular, my installation seems to crap out consistently on the line 
inside the data() function: 

$data = preg_replace("/([^\r]{1})\n/", "\\1\r\n", $data);


There are a couple of ways to solve this problem.  The best solution, which is 
probably the hardest, is to refactor the SMTP api to be more aware of file 
attachments and avoid doing the read-file/encode/write-to-network on the entire 
file.  If the SMTP layer was aware of file attachments, it could do the read-
file/encode/write-to-network on reasonably sized blocks.