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Document ID: 4654
Subject: Best practice for specifying a valid HELO and EHLO during email communication
Creation date: 12/14/15 4:35 PM
Last modified on: 1/8/20 5:24 AM

Valid HELO/EHLO identifier

Xeams checks for a valid HELO/EHLO identifier when assigning a score to emails. An incorrect value may result in an email marked incorrectly as spam.

Information for Email and Network Operators

Although email servers can (by RFC) accept connections that have a poorly formatted HELO or server identification string sent during email transmission dialog, many best practices documents insist that all identifiers are correctly used, and in the case of HELO (or EHLO) this applies as well. The principal is that the HELO should identify the sending server in such a way that it can be used to identify servers with problems, such as leaking spam or incorrectly formatted emails.

Xeams performs simple checks on the format of the HELO sent. It requires that the HELO (or EHLO) string that is sent is in the format of a fully qualified domain name (FQDN).

For example:
Additionally, Xeams also checks if you are sending this email from

The following entries are incorrect and will get rejected:
HELO localhost
HELO localhost.localdomain
HELO hostnameonly
HELO (just an IP)
HELO .com (starts with a period)
HELO @(&$ (characters not normally allowed in domain names)

Spammers will often be caught by this rule, when they take over a PC to act as a spam bot. They just use the hostname as the PC has it configured, which is normally not set up as a FQDN. Also, often administrators might install email server software without intent, that gets compromised or activated, and often it will use just 'localhost'.

Xeams does not apply this rule if the message is being sent by an authenticated user, local user or the IP address is open for relay or is white-listed.

Configuring your Xeams for outbound delivery

It is important you configure your Xeams correctly when sending outbound emails to other domains. A receiving server could reject your email if it is not configured correctly. You also run into a risk of getting your IP address blacklisted if your server is not configured correctly.

The following steps are required to setup Xeams for outbound delivery:

  • PTR Record - Ensure your public IP address has a reverse host name. This is called a PTR record in your DNS server. Let's consider as an example. The public IP address where synametrics email server is running is A PTR record has been added in the DNS server so this IP address resolves to An A lookup on this FQDN will return the same IP address.

    You will have to ask your ISP to add a PTR record, since they are the ones who owns your IP address.

  • SMTP HELO Domain - The host name you assigned to your PTR record must be entered in Xeams. Go to Server Configuration, select the Advanced tab and enter this FQDN in the SMTP HELO Domain and Smtp Signature fields

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