We're seeing a lot of errors like this which ranges from folks using their own email on their domain to yahoo, aol etc.

----- The following addresses had fatal errors ----- 
[Status: Error, Address: <XXXXXXX@yahoo.com>, ResponseCode 421, 4.7.0 [TSS04] Messages from 135.84.80.194 temporarily deferred due to user complaints - 4.16.55.1; see https://help.yahoo.com/kb/post master/SLN3434.html]

We are a tax and accounting firm so we are sending out time sensitive emails, can this be fixed? Our users are not receiving them.  Thank you