Messages in deferral will remain in our queues for 1 day. Message retry attempts are based on the error we get back from the recipient's mail system. The first few deferrals are 15 minutes or less, with subsequent retries (over the next half dozen or so) increasing the interval over multiple retries to a max of 60 minutes. The interval duration expansion is dynamic, taking into consideration multiple variables like queue sizes and internal message priority. In basic, it's 15 minutes (or less) to start, then expanding from there over the next few hours to 60 mins max.