I have noticed that my Ubuntu box is lately always trying to reach a SMTP server (detected through Wireshark tool). After reviewing the list of running processes that contains the word mail (running ps aux | grep mail on console), I found a couple of them called Nullmailer that seems the responsible of that and indeed, it was.
So a Google search led me to this thread, where in that case, Nullmailer registered entries on Syslog (which possible happened on my computer also). The messages that Nullmailer was trying to send were related to Cron jobs output. So, the solution was to delete the queue of messages of this program, located on /var/spool/nullmailer/queue/.