In this tutorial, you will learn the how to change the Divi contact form subject line

wp_mail( apply_filters( 'et_contact_page_email_to', $et_email_to ),
				et_get_safe_localization( sprintf(
					__( '%1$s has sent you a message %2$s', 'et_builder' ),
					sanitize_text_field( html_entity_decode( $contact_name, ENT_QUOTES, 'UTF-8' ) ),
					( '' !== $contact_email ? ' by ' . $contact_email : '' )
				) ),
				! empty( $email_message ) ? $email_message : ' ',
				apply_filters( 'et_contact_page_headers', $headers, $contact_name, $contact_email )
			);

If you’d like to further customize the Subject line, you can replace this piece of text with your own Subject:

%1$s has sent you a message %2$s