helo
also: SMTP HELO, Hello command
The SMTP command used by a mail client to identify itself to a mail server when initiating a connection; modern systems use EHLO instead.
HELO stands for "Hello" and is part of the Simple Mail Transfer Protocol (SMTP). When a mail client connects to an SMTP server to send email, it must first introduce itself using this command, followed by its hostname or domain name.
For example, a mail client might send: HELO mail.example.com to tell the server its identity. The server responds with a 250 code if the connection is accepted.
In modern email systems, EHLO (Extended HELO) has largely replaced HELO, as it allows clients and servers to negotiate additional features like encryption and authentication. However, HELO remains valid and is still used by older systems or as a fallback.