The same url you'd have in your browser when using the web-client.
Depending on if they use a balancer/proxy it may be like mail.domain.com or server.mail.domain.com etc.
Of course if their http/https connection is still down the desktop/offline client won't be able to connect and do initial sync - so you'll have to wait till it's up first.
Looks like they are online now...when the client opens here is the address I find;
When I enter the above info into the Incoming Server field I receive an error [the Incoming Server name turns to red].
Enter just mailbox15.lycos.com also might have to uncheck SSL (I see no HTTPS connection)