← Glossary
Authentication
What is HELO / EHLO?
The greeting a sending server uses at the start of an SMTP conversation to identify itself.
How it works in practice
The name it announces should be a real, fully-qualified hostname that resolves to the connecting IP and matches its reverse DNS. Some receivers reject greetings that are bare IPs, 'localhost' or names that don't resolve.
The common mistake
Running a mail server that greets with its internal machine name.
Related terms
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