Having enabled logging the message below is shown, despite the mail server being configured correctly, having tried recreating the settings, etc:
[confluence.mail.template.AbstractMailNotificationQueueItem] send Unable to send email since no mail server has been configured.
This feature was working fine, and 'spontaneously' stopped working one day - no changes to any config, no updates being applied, etc.
Any pointers or areas to look into would be greatly appreciated.