It appears that AT&T / Yahoo based email addresses might have occasional issues with TLS 1.0. So, if you use any email address ending with @sbcglobal.net, @att.net, @yahoo.com and the like, try using TLS 1.2 in Expert account properties - which might behave more stable than TLS 1.0 with these addresses. If it doesn't work you can always revert back to TLS 1.0. TLS 1.2 is more secure and more future-proof and in this case might cause more reliable connection to the server. But it does not work for all servers, only those which support it.

TLS 1.2 setting offers better security but does not work for all email servers.