HELO stands for Hello and it's the very first thing your mail server says when connecting to another server, it’s the initial greeting that introduces the sending machine by name. These days, most systems begin with EHLO, which is an extended version, and will fall back to HELO if necessary.
Think of it as an opening handshake. The receiving server uses this greeting to identify who’s contacting it and then decides how much trust to extend in return. That single line can play a major role in establishing, or undermining, your sender reputation.
S: 220 mx.example.net ESMTP readyC: EHLO mail.yourdomain.comS: 250-mx.example.net Hello mail.yourdomain.comS: 250-STARTTLSS: 250 SIZE 104857600HELO tells the receiving server who you are before you send anything else.
Mailbox providers and receiving servers start evaluating your technical identity the moment you connect. The HELO string is a central component of that evaluation. It should always be a stable, fully qualified domain name (FQDN), not just an IP address or something generic like “localhost.”
No single check operates alone, but together they determine just how much of your mail will successfully reach recipients’ inboxes.
Reputation is essentially a record of your sending history and identity. The HELO greeting adds to that record. Using a valid, consistent hostname projects attentiveness and reliability, while a carelessly configured HELO string signals risk.
If your trust score drops, you may encounter several obstacles such as throttling (where your emails are delivered more slowly), greylisting (temporary rejection of emails from previously unknown senders), or outright blocks (total rejection of your emails). You could also experience stricter spam filtering, even if your email content is acceptable.
Addressing HELO-related issues removes a key reason for servers to doubt your mail. While it won’t mask abusive behavior, it does reduce easily avoidable technical problems that can hurt your delivery rates.
Begin by identifying the IP that’s actually sending your mail, then confirm that the DNS loop and your server’s greeting are properly configured.
dig -x <IP> +short, take note of the hostname that appears.dig <hostname> A +short; it should give you the same IP address you started with.swaks or openssl s_client -starttls smtp to watch the transaction.If you find a break in the DNS loop at any step, resolve your DNS records before retesting. Don’t rush, DNS changes may take some time to propagate across systems.
Assign each sending IP a purposeful, resolvable hostname, meaning a specific and meaningful internet address that can be easily tracked and identified. Use a subdomain you control, such as smtp.example.com or mailout.example.com. Avoid using your actual IP address or generic placeholders as the HELO identifier.
Document the associations between your sending IPs and hostnames. Whenever you add or decommission an IP, update both forward and reverse DNS records in one planned change window.
SPF (Sender Policy Framework) primarily checks the envelope sender domain to ensure it’s authorized to send email for that domain. However, if the envelope sender value is missing, many mail servers will verify SPF using the HELO domain instead. If your HELO domain lacks an SPF record, you risk failing SPF checks during bounces and system notifications.
To prevent this, publish a simple SPF record for your HELO domain that explicitly authorizes the correct sending IPs or hostnames. Make sure the record is accurate and doesn't exceed DNS lookup limits. For more detailed guidance, refer to these SPF strategies for Google, Microsoft, and complex multi-domain environments.
DKIM and DMARC center on the domain visible in the “From” address, not the HELO domain. Nonetheless, spam filters assess all these signals together. Maintaining a valid HELO, properly aligned SPF, and genuine DKIM signatures builds a strong, trustworthy identity.
Your sending reputation improves when your technical identity and actual sending actions line up. A clean HELO name indicates that your system is well-maintained and conscientious about following good emailing practices. Consistent sending habits and real mailbox engagement further reinforce this positive impression.
This is where Mailwarm steps in. It manages controlled send/receive interactions across more than 1,000 carefully maintained inboxes. The service automates actions such as opening, replying, removing messages from spam, and marking emails as primary, in order to teach email providers that your mail is genuinely wanted.
Mailwarm focuses on maintaining optimal email deliverability rather than sending out mass marketing emails. These “warm-up” emails have a specific technical purpose: to establish a positive reputation attached to your sending infrastructure.
When delivery fails, SMTP error messages often call out HELO or DNS problems. Look for these common replies:
To resolve these issues, always address the DNS loop first. Then double-check that your MTA presents the corrected hostname every time it connects. Make sure to test the results with several major providers to confirm everything works reliably across the board.
Treat HELO as your first impression to the outside world. Make it accurate, consistent, and as predictable as possible, for deliverability, boring is better.
If you want a second opinion about the set up of your HELO, the handling of your DNS, and a review of your email warming strategies (gradually increasing the volume of emails sent to improve delivery rates), consider consulting with the email deliverability experts at mailadept. A quick review today could save you from frustrating email deliverability issues tomorrow.
EHLO is an enhanced version of the traditional HELO command, offering additional capabilities. Falling back to HELO suggests outdated practices and could hinder email deliverability and server trust.
The HELO string is paramount to your email identity; a flawed one can lead to severe deliverability issues. Misconfigured HELO can result in email throttling, greylisting, or outright rejection.
An FQDN ensures transparency and legitimacy to receiving servers, reducing the risk of suspicion. Using anything less, like an IP or generic name, casts doubt on your intentions and infrastructure.
Absolutely, mismatches between forward and reverse DNS settings can severely undermine your credibility. Aligning these is non-negotiable, as inconsistencies will red-flag you to spam filters.
Failure to align SPF records invites unwarranted scrutiny, potentially failing SPF checks. Ensure your HELO domain has the correct SPF records to prevent bounces and authentication failures.
Consistency in using the same HELO hostname is vital for a stable email reputation. Changing hostnames haphazardly suggests instability and risks greater scrutiny from mail servers.
HELO is your introduction, setting the stage for how receiving servers see you. An errant introduction causes irreversible damage to your trustworthiness before your message is even read.