postfix – google denying mail, even though sending from my server’s IP

I just setup up a postfix server, it works, I can receive mail and can read it with the mail command, now if I try to send to a gmail address, it says the following:

host said:
    550-5.7.1 (<redacted :)>) The IP you're using to send mail is not
    authorized to 550-5.7.1 send email directly to our servers. Please use the
    SMTP relay at your 550-5.7.1 service provider instead. Learn more at 550
    i4si3930481pgs.164 - gsmtp (in reply to end of DATA command)

now I am directly trying to send this mail from my server, which has the IP of my domain, and according to the webpage it sends me to:

Another alternative is to send mail through your own domain’s servers

soo I am extremely confused, I do not want to deal with my ISP at any point of time, it is a last resort, they took one whole year to open ports, I do not want to ask for SMTP relays now!

I think mail isn’t sending from port 587(I think this is the right port), how do I force it to?

is it that I have to set up an MX record? as It’s been working without one, and if I try, freenom(I’m broke, sorry) will spit out Error occured: Invalid value in dnsrecord

if anyone can let me know why it’s being blocked despite originating from the IP of the server, or you have a solution, please let me know

sorry, I’m a noob in web applications, and I barely have any experience in it!

thanks in advance, satyamedh! (I’ll be heading to sleep now)

edit: forgot to include this, but as of right now I AM NOT USING SSL/TLS TO SEND/RECIEVE THE MAILS!, do not know if it matters tho!