Getting this for a bunch of Yahoo addresses. Do you know if some of your servers got blacklisted? Diagnostic-Code: 4.7.0 [TSS04] Messages from 136.143.169.51 temporarily deferred due to unexpected volume or user complaints - 4.16.55.1; see https://postmaster.yahooinc.com/error-codes

Remote-MTA: dns; mta5.am0.yahoodns.net