Does your connection to that server cross over from internet provider's
equipment to another's before it reaches the server?  If so, such
connections to port 25 are often blocked to cause trouble for spammers.
In this case, you should ask free.fr if their outgoing email provider
listens for outgoing mail on any port other than port 25, and if so, where
to find their instructions for switching to the other port.  The
instructions
are probably somewhere on their web site if they allow this.  Just trying
all the ports is a very slow way to find out, since there are about 65,000
ports available, and it's unlikely that more than a few of them are
listening
for outgoing mail.