Exim Log Error: temporarily deferred due to unexpected volume or user complaints

Symptoms

You find that messages to Yahoo are not arriving at the destination email address.

Additionally, you find errors in /var/logs/exim_mainlog that are similar  to the following :

2020-04-28 22:08:03 1m1x23-2011cZ-MN H=mta7.am0.yahoodns.net [67.195.228.106]: SMTP error from remote mail server after pipelined MAIL FROM:<SENDER@senderdomain.com> SIZE=1758: 421 4.7.0 [TSS04] Messages from xxx.xxx.xxx.xxx temporarily deferred due to unexpected volume or user complaints - 4.16.55.1; see https://postmaster.verizonmedia.com/error-codes

Description 

This typically happens after your server has sent out a large volume of spam messages.

Resolution

• You must locate the source of the spam messages and prevent them from being sent. These  are useful guides for locating spam issues:
  

  How to find the source of spam emails

  Using cPanel's Mail Status Probe script to diagnose spam related issues 

• The following is a useful guide to preventing spam from being sent from your server:
  How can I prevent email abuse
• Once you are confident the spam problem has been resolved, preventing mail from leaving your server, visit the documentation link provided in the error message. This is reproduced here:
  Postmaster Error Codes

• On that page, you'll find a section labeled "Message Temporarily Deferred". You must expand that section and read the documentation provided. There is a link located there that you may use to open a support request if the issue persists.
  Please note that while the documentation at the link above provided is from Verizon,  it still applies to Yahoo because they use the same infrastructure.