All Email Send Through Postmark Are Queued on Postmark Server

The only documentation I could find from Postmark was this:

You may (or may not) notice a new status type. We’ve added the status In Queue to specify if the message is currently waiting to be sent by Postmark. Right now, this just means that we’re waiting to send it to the mail server, but very soon we will even open this up so you can see the status on the mail server as well. If volume spikes on our side or an ISP decides to limit connections, you will be able to see the status.

From https://postmarkapp.com/blog/tracking-sent-emails-and-content

My guess is that messages get queued if:

• Postmark itself is under heavy load and can't process the volume of messages
• Postmark decides that sending messages to particular domains/providers might trigger rate limits, so some messages get delayed to stick within these limits
• Your messages appear to be suspicious in some way, and so messages get queued for their team to review your account and uncap it. This could possibly happen if your bounce rate is too high, messages look spammy, or you're a new account that has a sudden spike in sending rates.

We had about 1,000 messages get placed "In Queue" recently, but they seemed to all get properly processed within 1-2 hours when we checked the activity pane again later.