This is a pretty frequent error on all email systems. Often cPanel based customers report this mail delivery warning, and they think the mail server is failing, or it is a hosting problem.
The exact error message would look like:
From: Mail Delivery System Mailer-Daemon@server.nixcp.com
Subject: Warning: message 1fQdkg-00023y-1r delayed 24 hours
This message was created automatically by mail delivery software.
A message that you sent has not yet been delivered to one or more of its
recipients after more than 24 hours on the queue on server.nixcp.com
The message identifier is: 1fQdkg-00023y-1r
The subject of the message is: Survey Concepts / Mailing
The date of the message is: Wed, 6 Jun 2018 16:05:42 -0300
The addresses to which the message has not yet been delivered are:
No action is required on your part. Delivery attempts will continue for some
time, and this warning may be repeated at intervals if the message remains
undelivered. Eventually the mail delivery software will give up, and when
that happens, the message will be returned to you.
Final-Recipient: rfc822; firstname.lastname@example.org
Analyzing the body of the bounce shows that this is not a problem on the sending server and that you only have to wait:
No action is required on your part. Delivery attempts will continue for some time
This kind of errors happens because the email destination server is not able to receive mail at that specific moment, it could be due to a server outage, network problem or general server error. The email will keep trying to be sent, and if is not able to reach the destination mail server within the next hours it will stop trying.