Ok, antispamcloud.com is a domain name operated by N-Able on their Mail Assure
product (they also use mtaroutes
asked, how is the email being sent out from your server? According to N-Able the "550 message submission is for authorised users only" message should only be generated under the following conditions:
I assume your mail client is sending out from your cPanel server (and you are not trying to send out via a Mail Assure system)?
Can you try a service such as Mail Tester
(where you just need to send an email to the email address shown, wait about a minute and then click "check your score") or MXToolbox's Deliverability
tester or the comprehensive CheckTLS TestSender
tool (or even RedSift's OnDmarc's service
- but they may send you marketing emails) to confirm that it isn't a problem with the email address you are sending to.
If it still fails, I'm going out on a limb and guessing that either your email client is configured to send out via N-Able (either deliberately , through an MX/IP address misconfiguration or because there's a forwarder/filter in place somewhere).