A deferred event, or deferral, is simply an event SendGrid has received back from the receiving server that tells us that the receiving server has temporarily limited access to their system. This is equivalent to a busy signal.
It does not mean that your message will not be delivered. Rather, it is a signal that your message will not be delivered immediately. This can happen for a variety of reasons. Some of the most common reasons a message might be deferred at an inbox provider are that the inbox provider is seeing too many spam complaints for your email that has already been delivered or that the receiving server is having technical issues at that time.
If a message to a recipient is deferred for more than 72 hours, that deferral will turn into a block. Otherwise, it will result in a delivery event once the message has been accepted by the receiving server.
To check these, navigate to Suppressions > Blocks (in Sendgrid).
Search for the email address of the client you are researching.
Listed next to each email address is the reason the email address was blocked (highlighted below in green). Depending on the reason for the block, it is possible that SendGrid settings will need to be updated or changes will need to be made to the client's server.
If a deferred email turns into a blocked email after 72 hours, SendGrid will give instructions on how to unblock this email address (highlighted below in blue).
Please note that Punchh may not be able to implement changes if they exist outside of Punchh.