Rainer, It may be that Francesco's account is fine, but that his server is having difficulty or yours is - or one of the "translation nodes" on the route is screwed up. "Permanent error" merely means that the system has tried to deliver a number of times over a specified time period. Temporary problems can cause that "permanent" message. The internet reroutes messages (and message packets) to get through, but if the translation seems good from a node, then the system is confused. (Translation means turning the domain name into the actual internet address - for instance my domain is Murphsays.com but the address is 209.235.215.243 - that was the genious of Timothy Berners-Lee in inventing the Web, along with his making HTML with the links and rich text). Your message to Francesco may be because your server can't reach arcetri.astro.it, or because arcetro can't find triboli. Either would give you the same error message.
Best, Jon
