451 4.2.0 Internal queueing error after sending email

I have Norton Security version 22.9.3.13 running on my Acer laptop under Windows 10.  I use Thunderbird for email.

Two or three times recently, after the "Email sent successfully" message a pop-up from Norton has come up with the title "451 4.2.0 Internal queueing error after sending email" and the details of the sender, recipient and subject of the email.

Checking my Sent box, the email appears there.

Why does this message appear?

Has the email actually been sent?

Will the email arrive at its destination?

What can be done, if anything, to prevent this happening in future?

Any guidance/help will be gratefully received.

Gordon

I have labelled this as Email scan as it seems to be the closest label there is.