421-4.7.28 Call for help with Google rate limit

[zsimaiof]

I reach out to you because i am facing a very big problem over the last month with my VPS and Google rate limit.

I have a VPS as a mail server and during the last 30 or so days i keep getting " SMTP error from remote mail server after end of data: 421-4.7.28 [XXX.XXX.XXX.XXX(censored vps ip) 15] Our system has detected an unusual rate of\n421-4.7.28 unsolicited mail originating from your IP address. To protect our\n421-4.7.28 users from spam, mail sent from your IP address has been temporarily\n421-4.7.28 rate limited. Please visit\n421-4.7.28 https://support.google.com/mail/?p=UnsolicitedRateLimitError  to\n421 4.7.28 review our Bulk Email Senders Guidelines. qb17-20020a1709077e9100b009b282f14575si2417138ejc.395 - gsmtp"


I have SPF ,DKIM and PTR published and my server has excellent reputation- it's been online using this IP for 4 years.
The server or the domain is NOT listed to any other blacklist!

The fact is that the problem started when an ERP program used the SMTP server to send about 100 emails (not only to gmail accounts) in a few minutes timespan.
But that was about 35+ days ago and we made sure it won't happen again!
Since then the rate limit comes and goes. For example we didn't have any noticable problem during the last 10 days and today's morning it came back again!
After a few hours it managed to deliver some queued gmail emails but now it's back on!!

I checked exim sending logs for any spam activity but couldn't find any. The mail queue is mostly empty apart from when the rate limit is triggered.
There is NO SIGNIFICANT emails volume being sent to Gmail / Google Workspace (about 70-80 per day).

I also registered at google postmaster tools ~30 days ago (verified the domain) but the dashboard data is empty.

Please help me . I have already opened two support cases with Google and also posted a couple of times at The Google community but no one responded.

That's why i found you through the community posts and your Youtube channel and i decided to write to you, asking for help or perhaps some tips.

Please help me because i don't know what else to do.

Thank you in advance.