You can urgently abort the email sending process by following the steps below.
Step 1: Disable the email account in the Email Account Manager . After the email account is disabled, the system will pause sending any emails using the specified account.

Open Email Account Manager

Disable the email account in the Email Account Manager
Step 2: Locate the unsent emails (with status "Waiting for send") in the Outbox of Email & Message Manager, then cancel sending for these emails.

Open Email & Message Manager

Select the emails to cancel in the Outbox, then click the Cancel Sending button to abort sending these emails
Step 3: Enable the email account in the Email Account Manager .
