If you have setup and configured your Email To Case to process incoming emails and generate cases for the email that is coming from your own domain, but not from others, you may possibly see the soft bounce message for emails that are not from your domain.
Please check if your email to case routing address in Salesforce is set to receive emails only from your domain.
In Classic View
In Lightning Experience View
Next, under Email-to-Case Settings click Edit next to the Routing Address in question and make sure the Accept Email From box is empty.
Note:
If this option is grayed out, you do not have On-demand Email to case enabled.
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