Cannot configure Mail Merge Email

Hello.

I am trying to set up the Mail Merge function.
So I went to Tools > Options > LibreOffice Writer > Mail Merge Email and entered the account settings for my email address (from work). When I press “Test settings” , I have the window opening, staying blank for ages (~10-165 mn) before eventually telling me it fails at finding the outgoing mail server with the following error trace ;

LibreOffice could not connect to the outgoing mail server. Check your system's settings and the settings in LibreOffice. Check the server name, the port and the secure connections settings
--
<class 'smtplib.SMTPServerDisconnected'>: Connection unexpectedly closed, traceback follows
  C:\Program Files (x86)\LibreOffice 5\program\python-core-3.3.0\lib\smtplib.py:375 in function getreply() [raise SMTPServerDisconnected("Connection unexpectedly closed")]
  C:\Program Files (x86)\LibreOffice 5\program\python-core-3.3.0\lib\smtplib.py:322 in function connect() [(code, msg) = self.getreply()]
  C:\Program Files (x86)\LibreOffice 5\program\python-core-3.3.0\lib\smtplib.py:241 in function __init__() [(code, msg) = self.connect(host, port)]
  C:\Program Files (x86)\LibreOffice 5\program\mailmerge.py:106 in function connect() [self.server = smtplib.SMTP(server, port,timeout=tout)]

I have double-checked the settings (which I am quite sure of since they are the settings of my email client - i.e. Thunderbird).
With Thunderbird, the connexion to the SMTP server is SSL/TLS to port 465. I have specified a separate authentication to the SMTP server.

Any idea to help me debugging this ?