An attempt to send mail message to yahoo servers fails.
The following error can be observed in the
Jan 22 12:09:40 mail postfix/error: E0A7610C004: to=, relay=none, delay=155944, delays=155944/0.02/0/0.02, dsn=4.7.1, status=deferred (delivery temporarily suspended: host mta6.am0.yahoodns.net[18.104.22.168] refused to talk to me: 421 4.7.1 [TS03] All messages from X.X.X.X will be permanently deferred; Retrying will NOT succeed. See http://postmaster.yahoo.com/421-ts03.html)
Yahoo blocked IP address.
According to the Yahoo knowledge base article the "421 4.7.1 [TS03] All messages from X.X.X.X will be permanently deferred" message indicates Yahoo is seeing a high volume of emails from your IP address, which is a characteristic of unsolicited, bulk emailing.
Contact Yahoo support to unblock IP address.