Thunderbird mail client saying smtp SSL cert is wrong

Good Day,

I have a domain mydomain.tld and an SSL Cert from Let'sEncrypt for www. and just the domain name. That works ok for browsing to the URL but when I try to send from thunderbird I get a cert error "This certificate belongs to a different site". When I look at the cert it states it is for www.mydomain.tld. The Mail server URL I am using is the one pre-configured as mail.mydomain.tld. Under Manage SSL I did apply it to Postfix and everything else while I was at it. I restart services, restarted the server but still no luck. I tried adding mail.mydomain.tld to the cert but of course LE doesn't like it if its not an actual virtual server. Any ideas?

thanks

Other Info: Ubuntu 14.04.5 LTS Virtualmin 5.04 Pro Webmin 1.801