Your correspondent says the email hasn’t been received yet
There are a number of reasons why emails sent by you are delayed:
Our systems haven’t been able to deliver your email to the recipient's email server yet because of a transient problem. Our system will send you a warning email after 2 hours of trying.
Our systems have delivered your email to the correspondent’s email server, but their email server hasn’t yet delivered the email to them.
How to fix
View the email in your sent folder and verify the email address is correct – for example, you may have a typo such as firstname.lastname@example.org instead of email@example.com – carefully check the spelling of the address and try sending the message again.
You can ask your email administrator to verify all of the above possible reasons by querying the log files.