sean I've updated the CloudFlare DNS records to add MX records for the sub-domain I am using for the forum. I've set up an email address in OpalStack (forwarded to an existing mailbox for now). I've tested I can send an email to the forum address from an external system.
The error in the Flarum logs is:
flarum.ERROR: Swift_TransportException: Expected response code 220 but got an empty response in /home/scp/mcw-forum/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php:447
Googling it turns up a lot of people with the same issue but I think the solution lies with the hosting provider mostly so the responses are generally not useful. Do you have any thoughts? Is there a PHP function I need to enable?