For each email you send, Transactional Email automatically tracks and records the SMTP response we receive from recipient mail servers. Some successfully delivered emails will include a "queued" notation in the SMTP response such as 250 OK; queued as 12345. This indicates that the email was delivered to the recipient as expected, but may require additional processing before it lands in the recipient's mailbox.
For example, most times Transactional Email can send email much faster than recipient servers can accept or process it. Most times, things like the time of day and overall email traffic to that ISP or recipient server can affect how quickly they can receive and process your email.