We have a Rails app used for support & help tickets.
It sends customers a confirmation email when they submit a request. It also sends an email when we reply to their request. It does not receive mail.
Within the last week, 25%+ of our customers stopped getting their replies. They think we aren't replying to their ticket (when we actually are).
I did a test with my @yahoo.com account and found this in the mail.log file:
Jul 9 16:11:45 bighelp postfix/smtp: BF673324365: host b.mx.mail.yahoo.com[18.104.22.168] said: 451 Message temporarily deferred -  (in reply to end of DATA command) Jul 9 16:11:45 bighelp postfix/smtp: BF673324365: to=<email@example.com>, relay=d.mx.mail.yahoo.com[22.214.171.124]:25, delay=0.73, delays=0.02/0.02/0.64/0.05, dsn=4.0.$
Other non-yahoo addresses are experiencing this as well.
The Rails app is running on Ubuntu and we are sending via:
ActionMailer::Base.delivery_method = :sendmail
I made sure the server isn't an open relay.
What else can I do to ensure most of our emails get through?