Can't send email using outlook express 5, error 23016

My ISP just recently changed mail servers and I reconfigured the new incoming mail server (pop3) and outgoing mailserver (smtp)settings. I can receive emails but when I try to send an email I get this error " mail could not be sent to account XXXXX explanation: a connection failure has accurred error -23016. I'm running OS 9.1 on a IMAC G3 with outlook express 5.0.6. I've deleted and set-up a new account and have tried checking " my outgoing server (smtp) requires authentication" and "use the same settings as my incoming mail server ". My incoming server (pop3) is set for port 110 and I've tried my outgoing server port on 28, 25 and 587. I've call my ISP and talked with their tech support guys and everything is set correctly on my side. Any ideas guys ?

Imac G3 Mac OS 9.1.x