HELO/EHLO are SMTP greetings that identify the sending server. Learn how misconfigured HELO/EHLO can trigger spam filters.
HELO and EHLO are the initial greeting commands used in SMTP when a sending server connects to a receiving server. EHLO is the modern version that also announces supported SMTP extensions.
These greetings include a hostname that helps identify the sending system. Poor configuration, invalid hostnames, or mismatches between HELO/EHLO and DNS can be seen as suspicious by some filters.
Most modern email providers handle minor issues, but correct configuration is still a hygiene best practice. Align your sending server hostname with valid DNS and keep your infrastructure consistent to reduce avoidable filtering.