smtp is suddenly blocked on my server

Hi there, Suddenly, my server is blocked sending an email via SMTP. Could you please help us on this issue asap? Our application cannot send email to the customer any more, it is an urgent issue. Please help us to fix it.


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From a technical perspective, to send emails these days, you need to set up:

  • Reverse DNS entry
  • Milter
  • DKIM
  • SPF entry
  • X509 certificate

If you have a DKIM registration, then Outlook and GMail will be sending emails, telling you why they reject emails. If you can’t telnet to port 25, then DO have screwed up.

Hi, The problem is that Digital Ocean approved us sending email since 2017. SUDDENLY our servers are blocked without any explanation until we raised the issue. So disappointed with your service.

Hi, my server cannot connect to or (on remote port 587). I don’t understand why do we need to check the port 25. Many thanks for your reply.

Hi @quanglehuy2911

It’s possible you’ve closed port 25. Can you please login to your server and run

netstat -tulpen | grep 25

Please provide us with the result here.

Additionally, please confirm what are you using to allow tcp/udp traffic, is it for example CSF or just plain Iptables?

Kind regards, Kalin D.