These errors come directly from the email provider's server. Due to recent updates done by email providers on professional Workspace accounts, accounts are automatically blocking the sending of suspicious looking emails, especially from third party software like Kennel Connection. This means you'll need to make changes to your organization's DNS settings in the email account. Adding SPF, DKIM and DMARC settings to your email's DNS should help prevent these issues from happening. SPF specifies the servers and domains that are authorized to send email on behalf of your organization. DKIM adds a digital signature to every outgoing message. DMARC lets you tell receiving servers what to do with outgoing messages from your organization that don't pass SPF or DKIM checks.

This is not something the Kennel Connection software controls. It depends on the security settings of your email account in order to send emails from your account properly. The following articles should help point your email administrator in the right direction for resolving these issues.

For Google Workspace users: Help prevent spoofing and spam with DKIM - Google Workspace Admin Help

For Office 365 users: How to use DKIM for email in your custom domain - Office 365 | Microsoft Learn