Who is your SMTP provider? It looks like the remote end may be closing the connection which may point to an issue or a change on their end. If they have any alternative configurations, e.g. using a different port for SSL vs TLS or vice versa then that might be something to try to narrow down the issue.
Something else to check is that your version of OpenSSL seems to be very new - it indicates the build date is 25th March 2021. Presumably this update would have happened on your server some time after that date and therefore may coincide with when your issue started. Whether that means there's a configuration change required or a breaking change in OpenSSL or a bug, I don't know, but it's perhaps something that can be looked into.
Ultimately, however, not much that seems like we can do from the XF side but hopefully one of those things narrows it down.