Got an Iphone or pad. Try turning them off. They do not have that connection limiting buffer in their imap settings and will continue to connect until the pool of connections is exhausted.

Does a reboot of your machine "fix" the issue, even if temporarily? While writing this it occurred to me that other applications looking at mail, like anti virus, might hold open connections that they do not close. A reboot would force those closed until you next got mail.

Of course, there is the possibility that ntworld only allows a single sign-in at a time. Not common these days, but still possible as the server admin get to set the number allowed

It doesn't help because I don't have any apples! But at the moment I don't seem to have the problem!