Hi Eoin,
Thanks for the update. Here is the basic descriptions and configuration requirements about both the method follows.
SMTP client submission:
• If your device or application can authenticate and send email using an Office 365 mailbox account, this is the recommended method.
• In this scenario, the device or application uses an email account to send email to recipients just like an email client.
• If you want to send email from a third-party hosted application or service, you must use SMTP client submission.
• If your device or application supports or requires user name and password authentication, consider the SMTP client submission method instead of SMTP relay.
SMTP relay:
• This method allows Office 365 to relay emails on your behalf by authenticating using your public IP address (or a certificate). This requires a connector to be set up for your Office 365 account. (Might be the complexity of configuring a connector, SMTP
client submission is the easiest over Office 365 SMTP relay).
• However, in this method your device or application can send email using any address (including ones that can't receive mail), as long as the address uses one of your Office 365 domains. The email address doesn’t need to be associated with an actual mailbox.
Moreover, you may check the
summary of table inside the article it will help you decide which one of these method options will meet your business needs.
If anything is unclear, please feel free to let us know and we will be glad to provide further assistance.
Best regards,
Shyamal