Unable to add SMTP host

I’m moving my Cyberpanel onto a new host that doesn’t allow outgoing port 25 on their VPSs. They have a mail relay that all outgoing mail must be relayed through. I thought I could just set this up in the SMTP hosts section of Email Marketing but it doesn’t appear to work.

Their relay accepts connections on 25,587, or 2525. If I try 25 or 587 the page just sits their “processing” forever (the little animation goes but nothing changes). If I try port 2525 I almost immediately get an error message saying the username/password combination doesn’t work.

I’m using the same relay to send email from within several Laravel apps on the same server and they are working find with the same user/password and any of the 3 available ports.

Can anyone suggest where I could start troubleshooting this as I can’t see any issues in the logs.