Humm...now we're back to Mail app? Thunderbird is a good alternative. You're messing with me, right?
There is NO mail here on macosx.com; just a message system.
As originally posted, you'll need, apparently, to request help from your eMail provider as to incoming & outgoing address settings.
If, as you stated: "My ISP have been worse than useless", then it's time to get a new one!