Resolved - 10/06/2020. All effected messages have now been resent.
Our mail service provider, Sendgrid, are currently reporting service disruption issues which may prevent some emails from being sent through PayHero.
You can keep an eye on their current service status here - https://status.sendgrid.com/
Once the incident is resolved, the PayHero team will identify any messages that were affected by the outage and automatically resend them for you - no actions are required from your end.