We still use Netscape 4.5 at work (please - DON'T ASK!). The one thing I always liked about Messenger is the nsnotify app. For those that don't know it, it's a little app thats part of Netscape and runs in your system tray to check your email, without having to have the email client itself open.
It's a brilliant idea, but no other email client maker has ever taken it up - I've looked all over, but the nearest I've ever been able to find is a little app called
Mailbox Verifier. The main problem seems to be that, although several apps like this have been written, they always start adding on capabilities until they wind up almost as email clients themselves - this defeats the (to me) main object, which is to have a small app in your system tray using negligible resources.
Mailbox Verifier is good; does the job very well, but doesn't integrate with my email client (Thunderbird) as well as I would like. Although clicking on it will open T'bird, I always have to select T'bird from a right-click list the first time after a boot. It's supposed to stop checking when the email client is open, but, in fact, carries on doing so, and it doesn't know when I download email it's flagged up until it checks for itself.
Just my observation... ;-)