Our servers usually retry every 1-2 hours, however this does not mean that if your server was down for an extended period of time, such as days, that all the queued messages will return to your server within 1-2 hours after it is back online. Depending on the number of messages queued for your domain and how fast your server can process e-mails we are forwarding it, the amount of time it takes to get all queued messages to you could be several hours. We cannot change the retry interval as it is a global setting for our entire network and we can't manually force delivery of queued messages.