What is HELO/EHLO?

HELO and EHLO are the “hello” commands used at the start of an SMTP session. They let the sending mail server introduce itself to the receiving mail server.

• HELO = the original SMTP greeting (legacy).
• EHLO = “Extended HELO” (modern). It also announces which SMTP features the sender supports (for example encryption).

Why HELO/EHLO matters for deliverability

Email providers (including Microsoft/Outlook, Gmail, etc.) use HELO/EHLO as one of many trust signals. A “clean” greeting helps your email look like it’s coming from a real, properly configured mail system instead of a suspicious or misconfigured server.

HELO/EHLO issues don’t always cause an immediate block by themselves, but they can contribute to:

• Higher spam filtering
• Temporary throttling (slowing down delivery)
• Reputation damage when combined with other problems (SPF/DKIM/DMARC failures, complaints, poor list quality)

How it works (simple example)

When your sending server connects to the recipient’s server, the conversation typically starts like this:

• Your server: EHLO mail.yourdomain.com
• Recipient server: “OK — here are the SMTP features I accept”
• Then the email is sent using SMTP commands such as MAIL FROM, RCPT TO and DATA.

Mail systems that send email at scale rely on a Message Transfer Agent (MTA) to handle these SMTP conversations correctly and consistently.

Common HELO/EHLO problems

• Generic greeting like “localhost” or “server123”
• Hostname doesn’t exist (no matching DNS record)
• Inconsistent identity (hostname doesn’t match what you use for sending/authentication)
• No encryption support (some receivers expect modern security capabilities)

Best practices

• Use a real hostname as your EHLO identity (example: mail.yourdomain.com).
• Keep your sending identity consistent with your authentication setup (SPF, DKIM, and DMARC).
• Use SMTP authentication and secure sending to reduce abuse risk: SMTP Authentication.
• If you’re sending transactional email (password resets, order confirmations, invoices), use a reliable SMTP relay so configuration stays consistent: Mailpro SMTP Relay.

HELO/EHLO vs SPF/DKIM/DMARC

HELO/EHLO is part of the SMTP “handshake”. It introduces the sending server. Authentication records verify that the sender is legitimate:

• SPF/DKIM/DMARC prove your domain is authorized to send email.
• HELO/EHLO helps the receiving server evaluate whether your mail server looks correctly configured.

Related terms

• SMTP
• Message Transfer Agent (MTA)
• MX Record
• Transactional Email

FAQ

Is EHLO better than HELO?

Yes. EHLO is the modern version and supports SMTP extensions. Most modern servers use EHLO by default.

Can HELO/EHLO cause Outlook blocks?

It can contribute, especially if the greeting looks suspicious or misconfigured, but blocks are usually caused by a combination of factors (authentication failures, reputation, complaints, and sending behavior). Start by verifying SPF/DKIM/DMARC and keeping sending consistent.

Do I need to configure HELO/EHLO if I use Mailpro?

If you send through Mailpro SMTP Relay, the SMTP server side is professionally configured. You mainly focus on your domain authentication (SPF/DKIM/DMARC) and your sending practices.