Matt, 

Visit https://help.yahoo.com/kb/postmaster/, probably clicking the “new sender application” option would trigger a ticket to yahoo. Someone there should be able to help. 

U 

On Sat, Jan 12, 2019 at 4:19 PM Matt Hoppes <mattlists@rivervalleyinternet.net> wrote:
Our customers who use yahoo.com e-mail addresses are saying they aren't
receiving invoices from our billing system.  I checked our mail logs and
I'm getting this:

Jan 12 16:11:34 account postfix/smtp[9802]: 1FA906C0E61: host
mta7.am0.yahoodns.net[98.136.101.117] said: 421 4.7.0 [TSS04] Messages
temporarily deferred due to user complaints - 4.16.55.1; see
https://help.yahoo.com/kb/postmaster/SLN3434.html (in reply to MAIL FROM
command)

The link suggests several things to do and we've waited over 48 hours
and still can't get invoices through.

The invoice server is definitely not compromised, and honestly I can't
imagine there would be enough complaints to trigger a global block of
that IP address, and we only have a small number of customers using
@epix.net e-mail addresses.

Thanks!