Account Engagement Test or Internal Email Was Not Received or Resulted in a Hard Bounce

To avoid this issue, have your IT team add your company's assigned Account Engagement IP address to their spam filter allowlist. See Finding Your Sending IP for help on finding your IP. 

Are you using the Lightning Email Content Builder?

In this builder, the testing methods reference the logged-in user as the sender of the test. If email address of logged-in user has domain that is not verified as sending domain in Account Engagement, then test email is sent from test@[yourverifieddomain] (If you have more than one verified sending domain, the first one in the domain management table is used.)

If you have no verified sending domain, the email isn’t sent.

The manner in which the tests are performed can also play a factor:
Test messages sent via the one-off "Send to Individual Emails" option in the email builder are not sent as multipart MIME messages that contain both an HTML and plain text version of the email content. Rather, they are sent as two individual messages. Many email systems regard this behavior as suspicious and will react accordingly to one or both of them. Email test proofs should not be used as a measurement for deliverability. Rather, we recommend sending to test lists for best results. 

Additionally, we've found that some systems (Microsoft in particular) do not like this sending behavior. In some cases, repeated sends through the individual testing method can lead to a 'graylisting' of the sender. You may experience temporarily deferment the emails, potentially for several hours. If you suspect you are encountering this issue, Salesforce Support can help to confirm or deny. 

Finally, using add-ons or integrations with Account Engagement alter which sending IP is used. This table explains which IP is used in different scenarios.