This usually points to a DNS issue

Symptoms

• Form-to-mail notification fail with an NDR and error "550 Relaying Denied"
• Emails sent from accounts hosted by Net24 fail with an NDR and error "550 Relaying Denied"

And the following conditions are true:

• Website is hosted by Klixo/Net24
• Email is hosted by another party (on a seperate network)

Cause

Net24's local DNS server/proxy is resolving email (MX) requests to a local mail server that is not configured or being used.

Resolution

Get Net24 to remove the domain from their local DNS Server. This will force servers inside the Net24 network to resolve addresses for that domain (including the mail server address) externally.