Relay.plus.net not accepting mail

[#1]

Has anyone ever hosted their own email server and had relay.plus.net set as the smarthost for a *.xyz domain?

I have a few "production" domains (*.co.uk) and have in the past tested this configuration and it has worked.

I've just purchased some *.xyz domains purely for testing and I get the below message when forwarding through relay.plus.net.

mx1.*****.co.uk is my email server, this is able to serve as the email server for other .co.uk domains.

Reporting-MTA: dns; mx1.*****.co.uk
X-Postcow-Queue-ID: 2166A148A92
X-Postcow-Sender: rfc822; ***@********.xyz
Arrival-Date: Mon,  5 Apr 2021 17:32:08 +0100 (BST)

Final-Recipient: rfc822; *********@gmail.com
Original-Recipient: rfc822;*********@gmail.com
Action: failed
Status: 5.0.0
Remote-MTA: dns; relay.plus.net
Diagnostic-Code: smtp; 550 <***@********.xyz> sender domain rejected
    (blacklisted by local policy)

[#2]

When sending through relay.plus.net as a smart host, are you authenticating using your account username and account password?

[#3]

I wasn't authenticating before for my .co.uk domains, literally only had the smarthost set.

I've just added my credentials and still get the error.

[#4]

Do you have a static or dynamic IP? Also business or residential account? If the latter I suspect that may be the reason as it’s possible you’d only be allowed to run an email server on a business connection

[#5]

Residential with a static IP.
As mentioned, my co.uk can use the relay.plus.net fine as the outgoing smart host.

[#6]

I'll also add, inbound works fine and the SPF record has been created to allow madafish and plus.net as the authorised senders.

[#7]

I just ran some manual tests via telnet and it would appear that unknown domains such as somethingrandom.somethingelserandom are rejected as an 'invalid domain (no MX or A record' as you might expect but anything.xyz results in the 'sender domain rejected' rejection so it looks like Plusnet are - as per the 'blacklisted by local policy' expansion - manually blocking sending from that TLD as a whole.

Perhaps there was a big spam issue at some point in the past for Plusnet and a knee-jerk response of blacklisting the entire domain is now leading to false rejections despite increased legitimate adoption of the namespace? Well, I'm assuming legitimate usage - perhaps you're in the company of a lot of spammers and fraudsters by buying into that namespace (I see Google were an early adopter so maybe that's a case in point! ;-)). If so then the issue with Plusnet could be a taste of things to come if it's an often-abused namespace and it might serve you well to pick another TLD to reside in. New, cheap, gTLDs attract the wrong kind of customer (the owners don't care - these gTLDs are set up primarily to print as much money as possible despite what claims they might make regarding helping expand the Internet).