It essentially means that the emailed quote cannot immediately be delivered, but it hasn’t been completely rejected by the receiving emails server. 

This is sometimes called a soft bounce. 

We will continue to try for 72 hours to deliver a deferred quote.

In most cases we see that the deferred emails are delivered in under 5 minutes. 

The best place to check to see if a quote has been delivered after being deferred is under the email history section on the quote.