Unable to send email to gmail account from my mail server

I've created a mail server and have setup a PTR record from the ISP to my static IP server. The mail server works well and am able to send and receive emails and send mails to many email addresses including outlook/hotmail. But I'm not able to send email to gmail accounts.

I get the following error. I've read through many posts and searched to the 3rd page of Google and am still not able to resolve it.

    host gmail-smtp-in.l.google.com[172.217.194.27] said:
    550-5.7.1 [116.90.224.43] 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
    5.7.1  https://support.google.com/mail/?p=NotAuthorizedError
    d18si26072098pls.58 - gsmtp (in reply to end of DATA command)

• I searched for blacklisting of my IP and found a few and have got them removed.
• I've configured rDns from my ISP and it resolves to my domain
• I've a valid SPF, DKIM and DMARC
• I have a static public IP with ports opened from my ISP
• I'm using tls/ssl

How do I solve this issue? How do I prove to google that my IP is a valid IP and I need to be able to send mails to gmail accounts?

Solution 1:

I had to ask ISP to give me a new clean IP. changing the IP fixed the issue.