which protocol are you using to send the message?

AIM.  I haven't yet tried the others.

Well, I couldn't reproduce this sending kopete to gaim or kopete to AIM. I
suppose that the person on the other side was using trillian?

yup, they're using Trillian.

I know a fix for this. 

CVS commit by mattr: 

Replace the " " with a regular space so trillian doesn't display 
" " 

CCMAIL: 80158-done@bugs.kde.org
CCMAIL: 54433@bugs.kde.org


  M +1 -0      aimcontact.cpp   1.49


--- kdenetwork/kopete/protocols/oscar/aim/aimcontact.cpp  #1.48:1.49
@@ -295,4 +295,5 @@ void AIMContact::slotSendMsg(KopeteMessa
                 finalMessage += "</FONT>";
         finalMessage += "</BODY></HTML>";
+        finalMessage.replace("&nbsp;", " ");
 
         // ===================================================================================

CVS commit by mattr: 

Backport the fix for 80158. Should be in KDE 3.2.3

CCMAIL: 80158@bugs.kde.org


  M +1 -0      aimcontact.cpp   1.43.4.1


--- kdenetwork/kopete/protocols/oscar/aim/aimcontact.cpp  #1.43:1.43.4.1
@@ -333,4 +333,5 @@ void AIMContact::slotSendMsg(KopeteMessa
                 finalMessage += "</FONT>";
         finalMessage += "</BODY></HTML>";
+        finalMessage.replace("&nbsp;", " ");
 
         // ===================================================================================