Email bouncing

Known Sender Forgery

Reason

This approach is a simple method in which only the most often abused / forged domains are detected and validated as coming from authorized to send sources. The net effect is to stop the worse cases - be it fake bank statements, viral loaded 'delivery' notifications, or even forged newsletters from your favorite online music site.

If you have received an NDR containing a link to this site please speak to your System Administrator or Email provider.

Strict Address Parsing

Reason

Email must be from a valid email address that is constructed according to the RFC of SMTP.

Email addresses should consist of two parts separated by the @ symbol. Other validation rules apply to the address and domain parts of the email.

Valid email address example...

  • Mail from - <name@domainname.com>

Examples of invalid email addresses...

  • Mail from - name@domainname.com
  • Mail from - <name>
  • Mail from -
  • Mail from - <name@192.168.0.1>
  • Mail from - <name@@domainname.com>
  • Mail from - <name@domain$.com>

If you have received an NDR containing a link to this site please speak to your System Administrator or Email provider.

Require Full Email Address

Reason

In order for our services to receive inbound emails they must be addresses to valid email addresses and must be constructed as such.

Email addresses should consist of two parts separated by the @ symbol. Other validation rules apply to the address and domain parts of the email.

An example of valid email construction would be...

  • name@domainname.com

Examples of invalid email addresses are...

  • Name
  • name@
  • name@domain name.com
  • @domainname.com
  • @domainname
  • Addresses with invalid characters...

If you have received an NDR containing a link to this site please speak to your System Administrator or Email provider.

Require HELO

Reason

Email servers communicate with each other by starting a conversation. Email best practice dictates that a helo must be provided to properly identify valid email servers.

Scripts used by spammers, Trojans and bots can sometimes not provide this and make this an easy way to identify non valid email servers attempting to send.

A correct email server communication starts as follows, HELO or EHLO can be used depending upon requirements...

  • 220-mail.server1.com
  • HELO mail.server2.com
  • 250 mail.server1.com
  • MAIL FROM: email@server2.com
  • .....

If you have received an NDR containing a link to this site please speak to your System Administrator or Email provider.

Valid HELO Required

Reason

In order for mail servers to be properly identified a vaild HELO is required to start a conversation. The HELO command should be followed by a fully qualified domain name.

Example of valid HELO...

  • HELO mail.domainname.com

Invalid examples would be as follows...

  • HELO
  • HELO localhost
  • HELO localhost.local
  • HELO 192.168.0.1
  • HELO .com
  • HELO a string of text
  • HELO &%^£$ (any characters that are not valid for a fully qualified domain name)

For email uses sending email via their SMTP server this rule does not apply as your email client will communicate with the mail server in a different way.

If you have received an NDR containing a link to this site please speak to your System Administrator or Email provider.

Reverse DNS

Reason

Reverse DNS is used as a mechanism to verify that a server sending an email into our services is a vaild email server.

Best practice dictates that all email servers have a reverse DNS/PTR record and all major email providers will reject emails without PTR/reverse DNS entries. If you are receiving this error you will already be unable to send emails to a vast number of addresses across the internet.

The PTR records should reflect the mail servers domain name and not the Internet providers IP address, i.e. adsl.54.67.124.223.isp.com as this makes the email server look like an infected server or PC.

PLEASE NOTE: Durning a temporary DNS outage emails will be rejected with a temporary 400 error which will set the sending server to retry and not a permanent 500 error.

If you have received an NDR containing a link to this site please speak to your System Administrator or Email provider.

Block Mail Servers using Dynamic IP Space

Reason

There are two main types of hosts on the internet, clients and servers.

Clients are those used by home user PCs and are provided with Dynamic IP addresses when they connect to the internet.

Servers are those used to provide services to the internet and will always have a Static IP address. Email servers fall into this category.

The vast majority of all spam email, upto 75%, is generated by infected PCs on Dynamic IP addresses.

This test checks the IP address of the host machine that connected to our mail servers in order to transfer an email and if found to be Dynamic will reject the email.

If you are running an email server please contact your ISP to make sure you have been provided with a Static IP address. This may incur a higher cost for your connectivity.

If you have received an NDR containing a link to this site please speak to your System Administrator or Email provider.

Block Address with Invalid Domain

Reason

By default email servers will accept messages from any domain, we have adopted best working practises to improve customer security by refusing email address with invalid domains.

Email servers will readily accept emails with invalid domain suffixes. This is the part of the email address after the @ sign.

Although some email servers will accept these emails they are not valid or acceptable. The email domain is the second part of the email address following the @ sign and should be a fully qualified domain name.

Examples of invalid email addresses are as follows and will be rejected...

  • fredblogs@192.168.0.1
  • fredblogs@text

An Example of a vaild email address is and will be accepted...

  • fredblogs@domainname.com

If you are a genuine sender and are receiving this message please check your email client config to make sure that you are sending from a valid email address.

If you have received an NDR containing a link to this site please speak to your System Administrator or Email provider.