[imp] Fetch Mail - size limit exceeded

[warrior23 at gmx.de]

In horde webmail there is a feature called fetch Mail to allow Other Mail
Accounts to be fetched. 
When I use this feature to import emails from other servers, I get an error
saying: 
The message "message-subject" from "sender-name <email at address.xyz>"
(6513266 bytes) exceeds fetch size limit. 
Importing smaller messages works fine.
 
I am using a Linux server with Plesk 8.4.
 
I have noticed there are two config files in /etc/psa-horde/imp:
conf.xml: In line 158 there is an option "size_limit" in the fetchmail
settings section. It was set to 4000000 (eq 4 Megabytes) and I have changed
it to 20 Megabytes.
conf.php: In line 35 there is a variable: $conf['fetchmail']['size_limit'] =
'20000000'; // this was originally 4 Megabytes, I have changed it to 20,
too.
conf.xml warns you, that the memory_limit setting in php.ini needs to be
more than double of this setting - it is set to 64 Megabytes.
 
I could not find anything useful in google, so hopefully someone here can
help me.
 
Any help would be greatly appreciated!
 
Thank you,
Chris