Our servers will attempt to redeliver each message on a specific interval over the time that your server is down. 

We will attempt a retry at 1,15,30,60,120,180 minutes after a message has been received and then 180 minutes every retry after that. 

By default, we do it 249 times for a total queue time of 30 days, 18 hours, 16 minutes. If your mail server comes back online and you are ready to accept all your email you can queue your messages via our Admin interface or you can contact customer care 24x7 and we can release your messages in a batch.