Apple definitely has some things to work out with this issue. Not only do they no longer have the Continue button available (which basically just auto-switched it to non-SSL) but they no longer have a way to accept the certificate when you view the SSL certificate details. This all tends to be related to SMTP, not incoming (but can work for both).
The issue usually comes from misaligned certificates or using the wrong type of security protocol with that certificate.
Potential ways to fix the issue:
1. change the authentication type to password (I think it tries to default to MD5 or something) and make sure you are using the proper username and password
2. use the ISP's actual mail server name (many shared hosting services will tell you your mail domain is "mail.yourdomain.com" but it's not that way on the certificate - it's usually "mail.theISPdomain.com" on the certificate, so it cannot validate.) ... if you do switch to this, you definitely will require your full email address as the username
3. Turn off all SSL until Apple addresses the issue.
@APPLE : you need to either allow the ability to accept the SSL certificate that is available on the server, or to continue and auto-switch to non-SSL as before. You are not going to force ISPs (especially shared hosting environments) to buy a certificate for each domain and it's not feasible for them to order multi-domain certificate when they don't know what domains they will host on a daily basis. You need to follow some modicum of accepted IT networking principals.
@APPLE : While we are on the subject of email ... you also need to stop this anal requirement of inputting incoming and outgoing credentials separately. 99.9% of all mail services are combined and use the same settings for both. You are the only OS and mail client that still requires separate entry, which is really confusing for many people. You should still have an Advanced section where this can be adjusted in the off chance it is required, but for most you should just be setting everything in one swoop during setup. The notion of separate inbound and outbound servers hasn't been used in decades now, except in specialized environments. You also do not require "extra SMTP servers". That was a very old requirement when ISPs used to only let people on their own network use their mail service. That type of validation (again) hasn't been used in decades, especially with the mobile universe we are now living in.