This also applies to Outlook 365 and Office 365 - basically follow this if using Microsoft to host your email.
For account administrators.
Log in to https://admin.microsoft.com
Click the icon in the top-left corner with three lines. Expand 'Users' and click 'Active Users'.
Click the username who you want to permit to send emails through Retreat Manager. Once the user is selected click 'Mail' in the pop-out on the right. Then click 'Manage email apps'.
Click all of the boxes. Then click Save Changes button at the bottom.
You must also disable security defaults in Microsoft Entra ID https://learn.microsoft.com/en-us/entra/fundamentals/security-defaults
The security defaults in Microsoft Entra ID block access through SMTP. Per the instructions at the above link:
Log in to https://entra.microsoft.com/
Browse to Identity> Overview > Properties.
Select Manage security defaults.
Set Security defaults to Disabled.
Select Save.
Continue with the steps in the Email Setup Tutorial: https://www.retreatportal.com/post/retreat-manager-email-setup