Usually, very few messages are kept in the queue, as they only need to pass through and are instantly forwarded to the recipient's mail server.
SMTP Router > Queue
"You have 2 main email queues: pending mails and deferred mails.
Deferred mails are messages that have been soft-failed and are being put on hold to try to be sent later.
Their status is Temporary failure.
By default, the SMTP service tries 5 minutes later.
Artica scan perdiodically the queue each 5 minutes in order to let you read the queue from this section.