A deferral occurs when an ISP or a mailbox provider is for some reason not receiving emails or the recipient mail provider has temporarily refused delivery of an email. Instead of blocking or bouncing the message, the provider will defer or temporarily delay receiving the message and wait for the email to be resent. This is also known as throttling.
There are several reasons for an email deferral, the most common one is the sender has send too many emails in a given time frame to a specific email service provider. Email providers can sometimes throttle incoming mail temporarily to stop the server from being flooded. This can also be done in order to check that the emails aren’t Spam.
Other reasons are similar include:
- recipient’s mailbox is full;
- receiving server doesn’t recognize your IP, and fears you might be sending spam; or
- receiving server doesn’t have any open ports to receive email.
Most ESPs will try to send the emails to the recipient for up to 72 hours, from the first deferral, until the email is delivered. Normally, it takes between a few minutes to a few hours until the email is delivered. If the email is still not delivered after 72 hours, the email addresses are placed on the Block Suppression list.
Suggested: Keep a regular track of your domain and IP blacklist status.