It's not a delay in sending but a delay in getting an acknowledgement from the addressees email server that the email was received.
And it can take time because some of these will not answer quickly enough.
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This number is calculated by subtracting the number of emails Delivered, Bounced, and Soft Bounced from the total number Sent.
When sending through Marketo, it's usually related to the recipient's ISP, but if you're noticing a large percentage of pending emails for extended periods of time every time you send emails, it might be time to revisit your deliverability strategy and check into your own ISP.