So cPanel configuration and DNS is irrelevant, as this plugin does not use PHP Mail or SMTP. It intercepts requests to wp_mail() stopping the email being sent via your webserver/email server. Instead it forwards the request over the Microsoft Graph REST API, using the Office 365 Account you register with the plugin to send the mail using Microsoft’s Servers. Your mail server and DNS/web server is therefore irrelevant.
I would start by looking back at the guide and making sure that your Azure Active Directory Account is using the same Microsoft/365 Account/Tenancy as the account you wish the mail to be sent from (i.e. your clients Office 365 Account). This is where you need to make sure you are setting the authentication app created within Azure Active Directory to only “allow accounts within this organizational directory only”. DON’T SELECT ANY OF THE OTHER OPTIONS.
Once you’ve done this, you need to make sure that when you click the authorize button, you are in fact authorising the correct account. To do this login to WordPress using an in-private/incognito browser window to force you to have to login using your client’s office 365 credentials. Providing the account is valid, you will get the success message within the plugins settings.
This will mean the plugin is able to send email via the API. Whether or not it will work with the form though is a separate issue. It sounds like the Elementor Contact form is not compatible. I would need a detailed PHP server log, or further information about the parse error to provide any insight as to why you’re getting the parse error. Without that I can’t be of much help I’m afraid.
If you do submit the log, make sure you remove any sensitive/personal info!