Sometimes, when sending or receiving email, a mail server might generate a cryptic message saying that an email couldn't be delivered, but it will retry.
The most common reason for this to occur is the recipient's mail server has implemented spam greylisting. Unrecognized mail servers are delayed for a short while. Usually you have to do nothing, and your email will continue to be delivered normally.
Please remember - Email is not a real-time messaging service. There are other apps for that!