Several people are giving differing Incoming and Outgoing "hostingplatform.com" thingies. The one I was given was:
Incoming Mail Server: mail0.hostingplatform.com
Outgoing Mail Server: mail0.hostingplatform.com
so it seems that the big difference is the single numeral digit. I actually tried mail6.hostingplatform.com and it did not work. Be that as it may, after deleting accounts and reinstalling them till I thought I would scream, I finally got it to work. Sort of. It still had trouble finding the server when sending outgoing mail. It would just stall out and sit there until I waited until the the dialog came up with Select the [highlighted] Server below button, and it would go through. This was still erratic and unacceptable, so I kept meditating and thinking till my head hurt and finally got the bright idea the the SSL Certificate was the fly in the ointment. I knew better than to hit up NetSol for any help, so I did some research about SSL which to me is all Greek. I finally figured out to go to Applications/Utilities/Keychain Access.app and launched it. I had NEVER done this before, so each step was carefully researched before I did anything I'd regret. Sure enough, I found under clicking on System Roots and Certificates (over to the left) there was the old Expired in October 2021 Certificate in the right hand column. Apparently, simply following NetSol's "instructions" did nothing to remove this Expired Certificate, so after researching deleting Expired Certificates I took the plunge and deleted it. And that was it—problem solved. If you scroll down to the "N"s you will find: Network Solutions Certificate Authority which is apparently the new one that Expires: Monday, December 31, 2029, but Mac Mail gets confused with two Certificates on board like it can't tell which one to obey. After deleting the old Certificate there is no longer a delay and a wondering if this thing is going to work this time and the outgoing email whooosh will be heard. I waited a couple days to post this just to be sure as I know LOTS of people are grappling with this.
As an aside, if you click on the "Expires" column you can see the expiration date of all the Certificates in Ascending order, and I see a couple set for Dec 15, 2021, so if things get wonky on a sort of Root level I know where I'm going to look first. Perhaps other providers of Certificates have a routine that does the housekeeping and Expired Certificates are automatically deleted but far be it from NetSol to actually do this for us... if fact, the Expired NetSol Certificate was the ONLY one I found amongst several scores of Certificates, so way to go guys.
I sooo hope this helps someone out there.