How to Configure the SMTP HELO/EHLO Name

When your mail server opens an SMTP connection it must introduce itself with a HELO or EHLO command followed by a hostname. That hostname is checked by receiving servers in several ways: they compare it against the connecting IP's reverse DNS (PTR record), they may verify it resolves forward again to the same IP (Forward-Confirmed Reverse DNS, FCrDNS), and they use it as a spam signal. A misconfigured HELO name is one of the fastest ways to land in the junk folder or get outright rejected. This guide covers setting the correct HELO/EHLO name in Postfix and Sendmail, wiring up the required DNS records, and verifying everything holds together.

How HELO/EHLO Affects Deliverability

The SMTP greeting looks like this on the wire:

EHLO mail.example.com

Receiving MTAs run several checks against that string:

• Syntax check – must be a fully-qualified domain name (FQDN) or an address literal. Bare hostnames like localhost or ubuntu fail immediately at strict receivers.
• Forward DNS check – the EHLO name must resolve to at least one A or AAAA record.
• FCrDNS check – the PTR record for your sending IP must point to a hostname that resolves back to the same IP. This is the one most often misconfigured on cloud VMs where the PTR is set by the hosting provider.
• Consistency check – many filters add penalty points when the EHLO name differs from the hostname in the Received: header or from the domain in the envelope MAIL FROM.

None of these are hard SMTP requirements under RFC 5321, but in practice major receivers (Gmail, Outlook, Yahoo) treat failures as strong spam signals.

Prerequisites: DNS Records First

Before touching MTA configuration, make sure your DNS is correct. Assume your server's public IP is 203.0.113.10 and you want the mail hostname to be mail.example.com.

A record

Add or confirm this in your DNS zone for example.com:

mail.example.com.  IN  A  203.0.113.10

PTR record (reverse DNS)

Log in to your hosting or IP provider's control panel and set the PTR for 203.0.113.10 to mail.example.com. On VPS providers this is usually under "Networking" or "IP Management". You cannot set PTR records in your own zone file unless you have been delegated the reverse zone.

Verify both records propagate

dig +short mail.example.com A
dig +short -x 203.0.113.10

Both commands should return each other's value. If they do not match, fix DNS before continuing — MTA configuration cannot compensate for broken FCrDNS.

Configuring the EHLO Name in Postfix

Postfix is the default MTA on most modern distributions. The EHLO name comes from myhostname in /etc/postfix/main.cf.

Set myhostname and related parameters

sudo postconf -e 'myhostname = mail.example.com'
sudo postconf -e 'mydomain = example.com'
sudo postconf -e 'myorigin = $mydomain'

myhostname is what Postfix uses verbatim in the EHLO banner. myorigin controls the domain appended to unqualified sender addresses — keeping it consistent with mydomain prevents envelope/EHLO mismatches.

Check the SMTP banner

The banner is also visible to receivers and should match:

sudo postconf smtpd_banner

The default is $myhostname ESMTP $mail_name, which is fine. Avoid advertising software versions to reduce fingerprinting:

sudo postconf -e 'smtpd_banner = $myhostname ESMTP'

Reload Postfix

sudo systemctl reload postfix

Verify the live EHLO

openssl s_client -starttls smtp -connect mail.example.com:587 2>/dev/null | head -20

Look for a line like 250-mail.example.com near the top of the output. That confirms what Postfix is advertising. You can also use plain telnet on port 25 if TLS is not yet configured:

telnet mail.example.com 25

Type EHLO testclient.example.com and the server should respond with 250-mail.example.com as the first capability line.

Configuring the EHLO Name in Sendmail

Sendmail is less common on new deployments but still present on some RHEL-family and legacy systems. Its EHLO name derives from the j macro, which is set from the system hostname unless overridden.

Set the hostname macro in sendmail.mc

Edit /etc/mail/sendmail.mc (Debian/Ubuntu: /etc/sendmail.mc). Find or add the Dj macro:

sudo grep -n 'Dj\|confDOMAIN_NAME' /etc/mail/sendmail.mc

If absent, add before the MAILER lines:

echo "define(\`confDOMAIN_NAME', \`mail.example.com')dnl" | \
  sudo tee -a /etc/mail/sendmail.mc

Regenerate sendmail.cf and restart

# Fedora/RHEL
sudo make -C /etc/mail
sudo systemctl restart sendmail

# Debian/Ubuntu
sudo sendmailconfig
sudo systemctl restart sendmail

Verify

telnet localhost 25

The greeting banner should show your FQDN. Sendmail also logs the EHLO value in /var/log/maillog (Fedora/RHEL) or /var/log/mail.log (Debian/Ubuntu) for every outbound connection.

System Hostname Alignment

Both Postfix and Sendmail fall back to the OS hostname when no explicit value is set. Keep it consistent to avoid surprises after reboots or config resets.

sudo hostnamectl set-hostname mail.example.com

Confirm:

hostnamectl status

The output should show Static hostname: mail.example.com. Also check /etc/hosts to ensure the FQDN maps to the correct IP and not just 127.0.1.1 (common on Ubuntu cloud images), which can confuse MTA hostname detection:

grep mail.example.com /etc/hosts

It should read:

203.0.113.10  mail.example.com  mail

End-to-End Verification

Use MXToolbox or mail-tester.com for a quick external check, but you can do it from the command line too.

Check what a remote server sees

swaks --to [email protected] --server mail.example.com --port 587 \
  --auth-user [email protected] --auth-password yourpass --tls

swaks (Swiss Army Knife for SMTP) is available in all major distro repos. It prints the full SMTP conversation including EHLO exchange so you can confirm exactly what name your server advertises.

Check FCrDNS with a one-liner

IP=203.0.113.10
PTR=$(dig +short -x $IP | sed 's/\.$//')
FWD=$(dig +short $PTR A)
echo "IP=$IP PTR=$PTR FWD=$FWD"
[ "$IP" = "$FWD" ] && echo "FCrDNS OK" || echo "FCrDNS FAIL"

Troubleshooting

• Postfix still shows old hostname after reload – Run sudo postfix check first; errors prevent config reloads from taking effect. Also verify with postconf myhostname that the value changed.
• PTR record not updating – PTR changes at cloud providers can take up to 24 hours and sometimes require a support ticket. Use dig @8.8.8.8 -x <IP> to bypass local resolver caching when checking propagation.
• FCrDNS passes but mail still bounced – Check SPF and DKIM. A correct EHLO/FCrDNS is necessary but not sufficient; most modern receivers also require a valid SPF record and a DKIM signature.
• Multiple outbound IPs – If your server has more than one public IP and Postfix selects the wrong source address, set smtp_bind_address = 203.0.113.10 in main.cf to pin outbound connections to the IP whose PTR matches myhostname.
• localhost or ubuntu appearing in EHLO – The OS hostname is not set to an FQDN. Run hostname -f; if it returns a bare name, fix it with hostnamectl as shown above and restart the MTA.