Misconfiguration of a hosted mail setup SMTP Authentication for relaying

[comaid]

Hi guys,
Yes, I think it is obvious that it will be my misconfiguration of the mail settings that has caused this error, but would anyone know where to start with this error, as there are too many trees for me to see the wood!!
Firstly, the problem is I send mail from MY domain account (<SourceDomain>) (hosted at RollerNet) using Outlook 365 and it is generally fine, apart from ONE DOMAIN (<DestinationDomain>) (also hosted at RollerNet), (BTW: same thing whether I use webmail or Outlook). The mail is returned with the following error:
For security reasons I've not posted the actual domains!
::
HEADER:
Reporting-MTA: dns; smtpauth.rollernet.us
X-Postfix-Queue-ID: DDE722800C2D
X-Postfix-Sender: rfc822; ITSupport@<SourceDomain>
Arrival-Date: Sun, 1 Aug 2021 14:50:43 -0700 (PDT)

Final-Recipient: rfc822; Admin@<DestinationDomain>
Original-Recipient: rfc822;Admin@<DestinationDomain>
Action: failed
Status: 5.0.0
Remote-MTA: dns; <DestinationDomain>
Diagnostic-Code: smtp; 550-Please turn on SMTP Authentication in your mail
client, or login to the 550-IMAP/POP3 server before sending your message.
smtpauth.rollernet.us 550-[208.79.240.5]:38982 is not permitted to relay
through this server without 550 authentication.

BODY:
This is the mail system at host smtpauth.rollernet.us.

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@<DestinationDomain>>: host <DestinationDomain>[23.235.200.239] said:
550-Please turn on SMTP Authentication in your mail client, or login to the
550-IMAP/POP3 server before sending your message. smtpauth.rollernet.us
550-[208.79.240.5]:38982 is not permitted to relay through this server
without 550 authentication. (in reply to RCPT TO command)

::

So the domain <DestinationDomain> is a NEW setup at RollerNet (so, not worked since we moved hosts and hanged all DNS settings), the <SourceDomain> was setup up years ago.
It seems that mail is being received by <DestinationDomain> from many other sources, just not from <SourceDomain> Tried using a yahoo webmail account to send mail to <DestinationDomain> and that works just fine.

Anyone have any pointers to get things rolling?

[comaid]

RESOLVED:
Maybe this could help others in future with similar issues.

We were moving from one host to another when our issues became apparent. So, OldHost account still operational and had all zone records set to local mailserver. NewHost configured to use RollerNet. Domains moved from OldRegistrar to NewRegistrar. NameServers set in OldRegistrar to point to OldHosting, hence local mail in our case. Upon domain transfer, NameServer settings carried over (as we wished, so we could leave mail intact on old server). We used Hover for the NewRegistrar and they have had issues with their Control Panel that displayed incorrect information with SOME domain extensions (it would have to be the ones WE use!!), they were also unaware of the errors until we pointed it out to them. The result was the nameservers although updated and awaiting propagation, showed up as the old nameservers, resulting in waiting for the panel to show it was fully updated, which delayed our changing records. We thought we'd be clever and check various websites to see if propagation had been accomplished. Funny bit here we learned was almost every AS showed the OldNameservers still recorded. After going back to Hover (because we thought the error was on their end) we looked elsewhere. It was while trying to edit SPF records and DKIM that another problem was evident. Armed with the correct SPF values (we knew they were), they were consistently being found to be in error! Further investigation of this error showed that at our Host, the OLDNameservers were in listed the Zone Records (only viewable from WHM). Once we changed the Zone records for the nameservers in WHM's Zone Records, the global nameservers took the next 48 hours to update fully and thus everything showed as being correct from that point onwards. We made the assumption that because the NS records at (https://dnschecker.org/#NS/) showed as incorrect, when mail was traversing the internet it used those records as opposed to finding out what the MX records were at my host. Who knew that lookups were pulled from cache which wasn't updated and never looked up the server of where the registrar said to look! The error noted in the OP's question was caused by the OldHost shutting down the account so SMTP authentication was not possible, which was unexpected as we didn't ask them to close it!
A case of not believing what you read or believing too much in what you do read!