well im have the same issue like others and coincidently we all started to have this issue just recently like few days ago. Please help us out as on the surface 2 mail app my Hotmail account ( The main account ) cannot send mails and on the account setting it says there is a problem with the server security certificate. So how to fix it ???

Does this issue only happen with Hotmail account? Have you tested the account in other mail programs or send a email via web mail in a browser? What is the result if we delete the account then recreate the account?

Please also refer to solutions in this link:

Supporting Windows Mail 8.1 in your organization

See this part Self-Signed Certificates in Windows Mail 8.1

http://blogs.technet.com/b/exchange/archive/2013/10/18/supporting-windows-mail-8-1-in-your-organization.aspx