First of all, an SMTP server is the machine that delivers the emails you send with a mail client. And any free email provider comes with an associated free SMTP server which takes care right of its delivery process: Gmail, for instance, uses gmail.smtp.com.
Now the main problem with a common free SMTP provider is that it cannot guarantee a proper deliverability of bulk emails – like newsletters – because it doesn’t use monitored IPs.
That is, you can end up sending your emails from a blacklisted IP or from an IP shared with a spammer – with dreadful consequences on your email delivery rate. (Also, free SMTPs come with restrictions about the number of messages or recipients that you can handle per day).