If you want to close out a pending email that is being sent but is stuck due to SMTP errors, you need to execute this script in SQL and delete it in the user's outbox in redis.
update CTEmail.dbo.Email
set InProcess = 0
where emailid = 1038239
It should synchronize with redis eventually, but to force the issue you can remove the CTS.email.outbox.1.<userID>
If the customer had an smtp issue and emails were needing to be sent out after their fix of SMTP side, you would need to run the below query. This script will set the sent count to 0 which means the email service will try to send it out again. The email service has a max send attempt config so if it reaches that max amount, that is the reason for needing to reinitialize the email send process.
update CTEmail.dbo.Email
set sentcount =0
where inprocess = 1