Can’t Send emails with mail.mchsi.com

I am using Windows 10 and Live Mail (from before I upgraded from 7 to 10).  My email worked fine for about a month after the upgrade - then out of the clear, cannot send emails.  My Internet provider (Mediacom) said my public IP was blacklisted due to spam and there was nothing they could do.  I ran Malwarebytes and full scan - only 28 tracking cookies found.  I changed outgoing and incoming to 465 and 995 (per their suggestion).  It did not work.  Later it did work.  Next day, it didn't work again.  Sometimes it's only blocking send.  Another time both send and receive. I've unplugged modem overnight to try and get new IP address.  Nope, same IP.  What the heck can I do.  I'm ready to just trash Live and use webmail from Mediacom (which I don't like).