Sending this text over Jabber:
   DNS Admin
   CacheNetworks LLC
   2002 W Chicago Ave
   Chicago, IL 60622 US

leads to sending this xhtml:
<html xmlns=''><body xmlns=''> Billing<br xmlns=''/>   DNS Admin<br xmlns=''/>   CacheNetworks LLC<br xmlns=''/><br xmlns=''/>   2002 W Chicago Ave<br xmlns=''/>   Chicago, IL 60622 US<br xmlns=''/>   +1.1111111111<br xmlns=''/><br xmlns=''/> </body></html>

which is then converted by gaim_markup_linkfy() in serv_got_im() to:
<html xmlns=''><body xmlns=''> Billing<br xmlns=''/>   DNS Admin<br xmlns=''/>   CacheNetworks LLC<br xmlns=''/>   <A HREF=""></A><br xmlns='<A HREF=""></A>'/>   2002 W Chicago Ave<br xmlns=''/>   Chicago, IL 60622 US<br xmlns=''/>   +1.1111111111<br xmlns=''/><br xmlns=''/></body></html>

Note the bolded area -- one of the xmlns tags got linkified and is therefore highly invalid.

I'm not sure what the solution is for Gaim... I don't want gaim_markup_linkify() running at all on messages being passed to Adium, and I just noticed this because a patch to keep it from being called wasn't applied to our libgaim build.