I got messages like that when there were changes at the SMTP servers. This particularly happened about a year ago when several of the ISPs serving my clients switched to requiring authentication for SMTP.

If authentication is required you need a pretty recent version of PMMail/2 because only the recent versions support authentication. The version I use here is 2.20.2382 and it supports authentication.

You can ask your ISP if he's changed the server or try loging into the SMTP server with telnet.