Form emails to Outlook not working

I have a contact form on a client site, and it's working fine (have swapped in my email which is Outlook 365 hosted) but form emails to the client's Outlook based email have stopped working a few days ago. 

I have whitelisted Squarespace's form email address, turned off POP (just in case they had a device somewhere that was downloading the emails before the client could get to them).

I've checked spam, focus and other views in Outlook.

Forwarding the form submissions from my email to client works fine.

I have manually typed in the client email address (just to make sure it wasn't a whitespace issue).

As mentioned the email notification works fine on my O365 hosted email.

I'm wondering if there's anything I've missed, or if there is an issue that is affecting deliverability between Squarespace and Outlook emails?

Thanks Paul - yep this was the issue!