i had some problems with large email attachment and solved
the problem with changing my .htaccess files in following folders:

1. basic install folder - .mydomain.com/roundcubemail/
2. config folder - .mydomain.com/roundcubemail/config/
3. log folder - .mydomain.com/roundcubemail/log/
4. temp folder - .mydomain.com/roundcubemail/temp/

.htaccess file is:
--------------------------------------------------
# AddDefaultCharset UTF-8
php_flag display_errors Off
php_flag log_errors On
php_value error_log logs/errors
php_value post_max_size 10M
php_value upload_max_filesize 10M
php_value max_input_time 10000
php_value memory_limit 50M

<FilesMatch "(\.inc|\~)$|^_">
Order allow,deny
Deny from all
</FilesMatch>

Order deny,allow
Allow from all
------------------------------------------------------------------------------------------------------
hope i can help somebody.... i can send files around 9MB and i use roundcube-rev327.tar.gz and php4.3
so long.....

i love it by the way. so long...

Thanks It helps !

I didn't checked this .htaccess file