Mail 'incoming mail' password problems

Hi everyone, I wonder if anyone can help with the settings in Apple mail on my G5 OSX 10.5.8?

I have recently opened a new email account but I am having difficulty with the settings in 'mail' I have set everything correctly but my 'incoming password' set on my G5 desktop is being declined. I have used the same settings on my G4 laptop, and my Mac pro with some difficulty but I got there in the end. I have also set it up on my iphone 4s without any problems. But my G5 no matter how many times I try, which is several, refuses point blank! I have tried both POP and IMAP, different servers advised by my web hosts, but all to no avail. I have had some limited success where it allows it for a few emails/moments both ways but then refuses. Also I have been able to send emails without a problem, but not able to receive. I have obviously checked all the settings numerous times, and they have been checked and agreed with by the web host. The problem appears to be the 'incoming password', which I have also tried changing for the email account. For some reason my G5 seems to be just sending the wrong password? I'm not sure if this can be an encryption problem? I have other email accounts on this computer and they are all working fine. I have tried deleting the 'keychain access' for the account and starting afresh, and even tried using a different keyboard to type in the password, to no avail? I did not have an easy time setting the G4 (OSX 10.5.8) and the Mac Pro (OSX 10.6.8) but they are working okay now. If I use the Outlook Web App on the same computer it logs on no problem. It is the apple 'mail' on the G5 OSX 10.5.8 which is causing the problem. I am drawing a blank on this one and would be grateful for any advice? Thanks.