Issue with outbound emails sent from website not being delivered

We have recently moved our email domain, samedaydispatch.co.uk from our website host 'Fasthosts' to Google Domains and have noticed that we are starting to receive undeliverable email notifications from postmaster@DSVR024488 which is the name of the dedicated server which hosts our website that we have with Fasthosts.

TECHNICAL INFO

Domain / DNS Provider: GOOGLE Domains
Website Host: Fasthosts (Dedicated Server)
OS: Windows Server 2012 R2

IIS

SMTP server configurations are available via the IIS Control Panel namely ‘Internet Information Services (IIS) 6.0 Manager’.
The website/webserver configurations are available via IIS Control Panel namely ‘Internet Information Services (IIS) Manager’.

SMTP Email Delivery

The default PHP Mail configurations are pointed to SMTP Virtual Server #1.
This server, configured for ‘mail.samedaydispatch.co.uk’, is responsible for routing all emails from the website to our O365 Exchange.

SMTP Credentials
There are none. No specific username or password is being used as the server is tied to IP: 77.68.40.223
(Only applications that are hosted at this address can send mail through SMTP Virtual Server #1).