In this case, be sure to write to our Technical Department.
Be sure to indicate in the message who was the sender of the e-mail, what address you will receive this message and (if you have such knowledge) - when the message was sent.
The Technical Department, for its part, will check the logs of receipt of this message and help diagnose the problem. Often the problem is the temporary unavailability of the Sender's or Recipient's server, the presence of the Sender's or Recipient's server's IP address on the blacklist, sometimes even anti-SPAM filters that you have configured yourself.
As you can see - the topic can be very complex, the more details you give us, the faster we will solve this puzzle.