Some ISPs and organisations have been blocking port 25/TCP to try to control spam relays for more than a decade now.
One of the obvious alternatives is hosting your private domain in an international mail server company that offers your service in alternate email ports to port 25, for instance Google/Gmail.
As @Stephen also correctly managed to point out before me, another alternative is using a VM somewhere else. Or having a partnership with some branch/exchanging services with a related institution outside Brazil.
Many commercial email services offer additional ports for their users, and there are also other commercial services to redirect email messages for a relatively small fee.
The latter option should be probably less costly than running a VM on the cloud, but can have security implications. As an example, SMTP Port - Port 25 block redirection service (I do not know them, just a random google example)
Another alternative is contracting the services of a commercial VPN, and sending the email through them. Again, the security implications of tunnelling your traffic through them are something to think about, and your SPF for instance, might have to be more relaxed or include their netblocks.
Google is no exception to having alternate ports to send email; one of the alternative "official" ports is often 587/TCP.
This table was the best I could come up with that documents their offerings and alternatives.
Google Apps SMTP settings to send mail from a printer, scanner, or app
The best option seems to be, if using Google, to have a Google Apps/domain with Google and configuring postfix to relay email via smtp-relay.gmail.com using port 465/TCP or 587/TCP as an alternative.
So to configure postfix for that, you have to add to your
relayhost = [smtp-relay.gmail.com]:587
I also remember I had to add to the Google apps domain admin interface the IP of the email server. Cannot remember at all where, it was not hard to find.
I would not also discount talking with your ISP to see if they:
- are offering a relay service for people on your situation, to configure in postfix;
- sell you a static IP address for a fee not subject to port 25/STMP control;
- and/or if they have higher tiers of services not subject to port 25/STMP filtering.
Their interest is in curbing zombies spewing spam, not preventing you from sending email.
Often the lower tiers/residential netblocks are blocked as a standard security practice, however the netblocks used by enterprise tiers are not subject to that blockade. (this is a first hand information, I also ran at a time an ISP in a Portuguese speaking country)