smtp is suddenly blocked on my server

August 16, 2019 221 views
DigitalOcean Applications CentOS

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.

4 Answers

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.

Hi, my server cannot connect to smtp.gmail.com or sttp.office365.com (on remote port 587). I don’t understand why do we need to check the port 25. Many thanks for your reply.

  • Hey @quanglehuy2911,

    A good first step here may be to check both port 25 & 587 as that is what you are having issues with, using the command that @Kdimitrov provided.

    You can also check the IP address of your Droplet with a mail blacklist tool to see if it may be the remote mail provider that is blocking your ability to send mail.

    https://mxtoolbox.com/blacklists.aspx

    Hope that helps!
    - Matt.

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.

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.

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