ALWAYS include your Vicidial version (full, not just the first 3 digits, LOL) and build. This is a requirement for all posts on this board.
postfix is already running and can be configured to send mail through many smtp providers. however, this is outside the scope of this board by a fair margin.
that being said, note that there are two basic pathways for email when being sent by a scripted process.
the first method is for the script to directly contact an smtp or imap provider, authenticate, and send the mail directly from the script. This is usually quite simple, but has a couple challenges: It slows down the script, since it must now wait for the completion of the mail process. And it requires each script in the system that generates email to have access to the "send" functionality and credentials.
the second method is for the script to use sendmail on the host server (in this case vicibox) to send the mail. then sendmail will accept the email immediately (allowing the script to continue immediately) and then process the mail with it's own smtp processor. this processor CAN send email directly to the recipient OR it can be configured to use an smtp relay service (generally with your ISP, and may or may not require credentials as you are on their physical network) or it can alternately be configured to authenticate with a paid or free public smtp or imap service such as gmail, godaddy, live.com, etc.
Note that some ISPs expressly block port 25 on outbound packets to avoid rogue email servers on their systems, thus forcing you to use Their smtp-relay or use a service running on another port. Usually this costs money, but gmail and several others will allow this for free if you have an account.
If you choose to use one of the relay-based (smtp or imap) services, negotiating the proper port and security method along with credentials can be a bit of a challenge and requires configuring your postfix or exim (or whatever service is running) very carefully. But ... that is outside the scope of this board and there is a great deal of help available for it. Do be sure you search for a help page/forum/etc that will have information specific to your version of postfix and likely your operating system version. Tends to make it more likely you will succeed. It is even more helpful if you can find configuration information specific to your outbound provider as well.
Do remember that an alternative is to allow the server to continue to send the mail directly to the recipient (if your ISP will allow outbound port 25). This will require a properly formated SPF record. SPF records do NOT require an FQDN. The SPF configuration line can explicitly include an IPv4 ip address as easily as a domain.
Happy Hunting