Have not seen this error message myself so curious as to whether it explains in "plain English" what went wrnog.
I would have thought the number of users getting similar "cannot reach host" messages would not be exceeded just because someone was using Windows 7.
Lots of possible reasons for not being able to connect, but in your case, the outbound mail should have been 'protected' from access by anyone on a remote service (unless they specifically support it).
Most ISPs allow any customer to send, generally with any 'From' mail address, because they only need to know the connection is part of their network (which is easy to check because the ISP allocates their IP address).
Your old ISP allowed you to access their SMTP (sending) mail server (presumably because they had not deleted your user details, allowing server to think you were an existing customer, but "on holiday abroad" or whatever).
When they finally deleted you from their 'current users' list, the SMTP server started blocking your attempts to send mail.