I thought that you meant what you said, but I wanted to see , exactly, what you had to say.
In first place, except for the several outages, which I have not experienced, lately, any @netscape.net account should be accessible from netscape.com|Mail button or you should have
a login page. This is not an IMAP or POP set up, and it is done directly from the website.
I believe that 30 days of non use is more like it, but 90 days are possible, before the account is considered dormant. Everything is wiped clean, but, as you said you can resume the use of the account.
The IMAP set up, in the mail client, came into use with Netscape6.x, and was usable, with fits and starts through the 7 series (The fits and starts tried my patience, and I gave up that set up). Since AOL owns Netscape, they thought that it was convenient to allow that special set up, by way of NS6/7 built in IM.
If you, indeed, were using a @netscape.net account with the set up in Netscape6.x/7.x, it would not have anything to do with your ISP. In mail, you have a very simplified set up, which does not require that you enter any server settings. All you need is your program, and this set up was not available in any other mail client. Only in the Netscape6/7 series.
Is it available now, I don't know, because I gave up on it, and, I gave up on Netscape7.2, so I have no way to check.
It was never possible to set up a @netscape.net account in the POP format, unless you used a third party application,
like freePOPs. I can use that set up in any mail client (I have Thunderbird and Seamonkey), although it is not so straightforward. And, I could receive mail, but I would have to send with my ISP SMTP, which allows that.
So, if you tried to access your @netscape.net account only from the mail client, it was the wrong way to do it. The correct approach would be to access it from the website (such accounts referred to as "webmail" accounts for a good reason. If you had problem accessing your mail at the website, it is another story. But, your ISP has nothing to do with it.
My explanation may not be of any help, but I would appreciate if you would confirm what you were doing, i.e. mail client only, or a mix of mail client and website.
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.13) Gecko/20060410 Firefox/1.0.8