- Emails for particular recipients reject with:
CONFIG_TEXT: 550 Your HELO/EHLO must be contain FQDN or literal address(RFC-2821)
Hostname of server contains "localhost" substring
- Recipient has Exim SMTP server:
# nmap mail.example.com -p25 -sV
Starting Nmap 6.47 ( http://nmap.org ) at 2017-09-03 11:50 +07
Nmap scan report for mail.example.com (203.0.113.2)
Host is up (0.42s latency).
rDNS record for 203.0.113.2: example.com
PORT STATE SERVICE VERSION
25/tcp open smtp Exim smtpd
Service detection performed. Please report any incorrect results at http://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 16.09 seconds
By default, Exim server rejects emails from the server which hostname contains "localhost" substring
Change server hostname in order to exclude "localhost" substring: Plesk > Tools & Settings > Server Settings > Full hostname.
Also it's possible to contact Exim mail provider in order to adjust default configuration of mail server.