When an email is sent, the mailbox provider's response of "Accepted" only means that they have received it and are processing it. The actual delivery of the email to the inbox depends on various factors such as reputation, engagement levels, and content filters, which can lead to significant delays in delivery. Inbox providers prioritize emails based on these criteria, with transactional emails like password resets receiving faster delivery due to their high engagement rates. While Time-to-acceptance is easily measurable, Time-to-inbox is a more accurate measure of recipient experience, but is outside of the sender's direct control. To build a good reputation and improve delivery rates, senders must closely monitor their sending habits and engagement levels.