Certain ISPs and mail providers have extremely strict criteria for accepting email, and will often defer email if they think it might be spam or if they have a lot of emails coming in at the same time.
The email will be eventually sent through, as our mail servers will keep attempting to deliver your message, but it may automatically appear in your recipient’s Spam or Junk folder once it is finally sent through.
If you are receiving a lot of these messages from one particular ISP or mail provider, you may want to consider seeing if it is possible for your recipient to switch to another ISP or mail provider. This may not be possible in all situations, so you may have to accept the de-prioritising if you wish to communicate with those recipients.