Over the last few days we can't send emails to Yahoo addresses , they are returned with with error messages like these:

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/postmaster/SLN3434.html

ERROR_CODE :553, ERROR_CODE :5.7.2 [TSS09] All messages from 136.143.188.53 will be permanently deferred; Retrying will NOT succeed. See https://help.yahoo.com/kb/postmaster/SLN3436.html

It looks like Zoho's IP addresses are blocked by Yahoo.