Why is the send list showing "email queued"?

The reason behind not being able to send email and receiving "email queued" can be many but One can also be that If you have enabled the Enforce Email Privacy Settings under the "Deliverability" option in Salesforce, you cannot send emails from Contacts, Leads, Person Accounts, and Users. To verify this, follow the steps below:

From Setup, enter Deliverability in the Quick Find box, and click on it. In the Data Protection and Privacy section, select Enforce email privacy settings.

Click Save. Go to a Contact, Lead, Person Account, or a User record. Click the email action on the record. The user will get the error "You are restricted from sending email." Resolution :- To fix this issue, you can disable the Enforce Email Privacy Settings or you can add the individual record to Contacts, Leads, Person Accounts, and Users