unable to send mail

[jhausser@tuxfamily.org]

Hello,

I recently installed IMP 2.2.3 (debian package) on a Debian 2.2 box with 
Apache/1.3.9, PHP 3.0.18, MySQL 3.22.30 and horde 1.2.4-cvs.

I can log in, receive and read my mail but when I compose and clik on 
the 'send' button, I get the following error message :

Warning: File Upload Error: Boundary length exceeds limit. 
in /usr/share/horde/imp/compose.php3 on line 71

and the message isn't send.

I check the max_upload_size setting in php3.ini but it is set to 2.5 Mo.

Any idea ?
Thanks in advance.