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

First, you must locate the source of the spam messages and then prevent your server from sending additional spam messages.

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 

And this is  a useful guide for preventing the issue from occurring again:

How can I prevent email abuse

Once  you are fully confident that you have resolved the spam problem that is originating from your server, you should visit the documentation link provided within  the error message that is noted  in the symptoms section above. That link is also included below:

https://postmaster.verizonmedia.com/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.

NOTE: While the documentation at the link above  provided is from Verizon,  it still applies to Yahoo because they use the same infrastructure .