[imp] Couldn't resolve host 'data:image' with html image in signature ?

[norbi]

Hi List,

our customer has setup inline picture in his signature via horde/imp.
If i try to send html e-mails over this account, it fails allways with 
"Couldn't resolve host 'data:image'"

the HTML-Code:

<img alt="" src="data:image/png;base64,iVBORw0KGgoAAAANSU.....
.....
=" />

Picture seems realybig :(

Horde 5.04/Imp 6.04.

How can i fix that ?

Norbert