Troubleshooting Email Fail Delivery To an External Recipient Issue

1. Objective


In this article will give some ideas to fix the problem that an email was sent but it is fail to deliver to an external recipient email.

2. Problem Description


An internal user complains, while the rest is fine,  that she/he had sent an email to a person who is working in another company and she/he have never got the reply from him/her. The internal user calls to check with that external person and got the reply the email have never reached him/her.

3. Problem Resolution


  • Check Internal Spam Firewall

The issue may happen somewhere along the way from our internal mail server to that affected external company mail server. Check on our internal spam firewall to make sure that the email had already delivered out from our internal spam firewall.

  • Check IP/Domain Blacklist

It is also a problem if the public IP addresses that is NATed for mail server of both sites are in black list. We can check it with this website Make sure that we get the green check for each item.

  • Check IP Reputation

There is also a case that the public IP addresses that is NATed for mail server of both sites are in a bad reputation. We check this with this website

  • Check If the Existence Of the External Email Account

Sometime our internal user may enter the external recipient email incorrectly or that email address does not really exist. With this link we can check if an email address is correct or it is really exist.

  • Check If the Email Is Working Properly

It could also possible that the email address is correct and really exist. However, it could not receive an external email address. To check if an email could receive an external email address, use the this link and go to option “Email Tests”.


  • Check With External Company IT Staff

    • Check attachment file size setting:  when an internal user sends email with an attachment file that is over limited size setting of   the external recipient email, the email will never reach the external recipient email inbox.
    • Check available space of the affected Email: when the external recipient mailbox is full, it will not receive any incoming mail.
  • Test send/reply with IT Email: send a test email to IT email address and ask him/her to reply it to make sure this problem happens to all of the external company email address or just a specific email address.
  • White list our email and test send/reply again: the sent email may stop at spam firewall of the external company. So, we can by white list our internal mail addresses and try to to send and reply again.

That is what i have done so far. If you have another way that contribute to the resolution of this problem,  it is really appreciated if you could share it with the command below.