Most probably it is empty because it was “rotated” by the server and no other tests were made in the mean time.
Could you please make a new test with your mail client and check if there are any new SMTP-IN lines in smtp.txt (or other *.txt file present into the log folder)?
Generally if something goes wrong while mail is being send we would see an exception being thrown. But here in this case the client gets stuck which means that’s something wrong with the configuration of server rather than server side code(this I’m claiming based on previous experience with non-smtp setup when firewall was disabled I could recieve the mail).
We are not on the same page here as you are writing your own mail client so you should be able to debug it on your side in order to point where the problem is (if any).
We could continue our investigations if you could use the standard SSL client openssl (you should find a way to install it on your system - you may find some hints here) and show us the output from the following command: