Question Mail issues: Undelivered Mail Returned to Sender root@s21209059.localdomain

[Christopher-James]

Hi

Im getting 5+ emails below from our PLESK install from email root@s21209059.localdomain.

What is sending the mails and how can i change the sender address so it wont keep failing.

Full email below, always the same.

Thanks

Undelivered Mail Returned to Sender

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Plesk Onyx
Version 17.8.11 Update #10

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Undelivered Mail Returned to Sender

This is the mail system at host s21209059.localdomain.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

The mail system

<admin@developer.saltmedia.co.uk> (expanded from <root>): host
mx00.1and1.co.uk[212.227.15.41] said: 550-Requested action not taken:
mailbox unavailable 550 invalid DNS MX or A/AAAA resource record (in reply
to MAIL FROM command)
Reporting-MTA: dns; s21209059.localdomain
X-Postfix-Queue-ID: 6502420BA7
X-Postfix-Sender: rfc822; root@s21209059.localdomain
Arrival-Date: Mon, 11 Jun 2018 12:08:04 +0100 (BST)

Final-Recipient: rfc822; admin@developer.saltmedia.co.uk
Original-Recipient: rfc822; root
Action: failed
Status: 5.0.0
Remote-MTA: dns; mx00.1and1.co.uk
Diagnostic-Code: smtp; 550-Requested action not taken: mailbox unavailable 550
invalid DNS MX or A/AAAA resource record

From: "(Cron Daemon)" <root@s21209059.localdomain>
Subject: Cron <root@s21209059> /usr/bin/kcarectl --auto-update --gradual-rollout=auto
Date: 11 June 2018 at 12:08:04 BST
To: root@s21209059.localdomain


The IP xxx.xxx.xxx.xxx was already used for trialing on 2018-05-09



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[Monty]

The hostname of your server currently is "s21209059.localdomain", so any script (for example a cronjob) that produces an output will use this hostname to send mails which obviously won't work as this hostname cannot be resolved.

Solution: Set your hostname to a name that is registered in the DNS. Here are some references you can use:
How to change or get a server hostname on Plesk server
HowTo/ChangeHostname - Debian Wiki