Thank you all.
I take comfort in knowing that I am not alone in my frustration at the disruption caused by El Capitan to one of my email accounts.
I spoke with AppleCare and they were again useless at resolving the issue I needed help with! They have a default setting of stating it's nothing to do with them but to speak with your ISP.
I deleted altogther the POP account that wasn't receiving/sending mail in the hope I could restablish it using the same information as before but now I get the 'Uable to verify account or password'. AppleCare said this proves that the issue is with my ISP as other POP accounts are working fine.
The annoying thing is that the email account in question is working fine on my iPhone and iPad. It was working fine on my 27" iMac until I upgraded/downgraded to El Capitan.
Apple not accepting responsibility is no doubt a legal thing. If they admitted it was their issue I suspect there would be a Class Action against them.
The irritating thing is that the email account that is now not working is my professional one and this causes issues.
I have attempted to resolve this issue by following the advice given in the various posts regarding the loss of email accounts but to no avail. having deleted the actual POP account on my system prevents me from doing some of the other things people have suggested.
So I'm in a state of limbo. I hope that my ISP might be able to offer some insight but I doubt it.
If anyone reading this has any other solutions to creating a POP account on my iMac using the settings that apply on iPad/iPhone, please let me know.
But in the meantime, joining in the condemnation of Apple may not resolve my mail issue but it feels bloody good!