[imp] Can't send mail with attachment

[Iohann]

Hi all.

My problem is that IMP does not send mail with attachments > 1 Mb. Attachmnet uploads to /tmp directory into temporary file without any errors, but after preesing "Send" button  blank "New message" window stays on the screen and none mail is sent. I try IE and Opera browsers. With attachment < 1 Mb I have no problem, all works fine. With mail client (MS Outlook Express) I can send any attachment.
Can anyone help me?
My setup is:
FreeBSD 6.1
postfix 2.3.3
apache 1.3.37
php 4.4.4
horde 3.1.3
imp 4.1.3

Thanks.