[imp] Big Attachment Size Problem

[Robby Sirwaturai]

I've search this mailing list, but i haven't found solution for my problem.
Or I missed the solution?

I'm using :
Red Hat Linux 9.0
Apache 2.0.40-21
PHP 4.2.2-17
Mysql 3.23.54.a-11
Horde 2.0
IMP 3.0
Avg. free physical memory = 40M
Avg. free swap memory = 1.4G

/etc/php.ini
max_execution_time = 3500
memory_limit =32M
post_max_size = 16M
file_uploads = On
upload_tmp_dir = /tmp
upload_max_filesize = 8M

/etc/httpd/conf.d/php.conf
LoadModule php4_module modules/libphp4.so
AddType application/x-httpd-php .php .phtml (I've tried the <Files *.php>
model. Both of it works fine, but i prefer AddType model)

Here is the symptoms: 
I can upload/attach file < 8M, the file appear in attachment list and /tmp
folder. But less than 5 second after I press the Send button, I get HTTP
error 64 - Host not available. But if I send files sized < 3M, there is no
error shown up.
There are nothing in httpd error_log, nor at the imp error_log.

Anyone can help me?? What I've missed?

Best regards,
Robby Sirwaturai