Hi,
Have you check: "Settings" > "Branding" > "System generated emails from address"? Make sure you have there your own custom email address.
Anyway I notice this also happens after CP installation, when defining Platform > System Emails > Test, it will send using the "noreply@enhancecp.co.uk" address, what will most likely fail on the SMTP relay.
This happens even if in Settings > Branding > System generated emails from address > is set a custom email address.
This should be fixed, either by:
a) When making the email test, a place to define the FROM address for the test
b) Make Enhance always send FROM the defined custom email in: Settings > Branding > System generated emails from address