Microsoft Outlook Express (OE) is an obsolete mail client that was available in Microsoft Windows XP, Windows 2003 Server and older Microsoft operating systems. It was no longer available on Windows Vista and later, though Windows Live Mail is relatively close in user interface and appearance.
Despite being obsolete and only working on operating systems no longer supported or updated by Microsoft, it still has some users who prefer its simple but powerful user interface. Some of those users will have had a frustrating experience recently, when various mail servers stopped working for outbound mail in OE. Specifically, these are mail servers that use SSL on submission port 465 or 587 for SMTP.
Secure Socket Layer (SSL) is a mechanism for encrypting data between a client and a server. You may know it from website URIs starting with “https:” and web sessions displaying a padlock symbol next to the URI. There are various protocol versions that can implement this encryption layer. One of these, TLS 1.0 which was conceived in 1999, has now been officially deprecated (made officially obsolete) as of the end of June 2018. Software now has to use more recent protocols, such as TLS 1.1, TLS 1.2 or the recently defined TLS 1.3.
Unfortunately, TLS 1.0 is all that OE will speak. It does not understand TLS 1.1 or later. Therefore it can not pick up mail from a POP server using SSL on port 995 or an IMAP server on port 993 or send mail to an SMTP server on port 465 (or 587) with SSL enabled.
The only workaround I am aware of (other than switching to a more modern mail client) is to use Stunnel, a tool for Windows or Linux that acts as a proxy. You can configure it to establish an SSL connection to a given host and port when a connection to a given local port is made. Thus you could configure OE to connect to port 9465 on the machine running Stunnel, which might then connect via SSL to smtp.example.com:465 using a more modern TLS version supported by Stunnel (but not directly by OE).