why does some outgoing mail constantly end up "Deferred" ?

[William Dudley]

This, in /var/log/maillog:

Apr 14 13:17:52 dudley sm-mta[13944]: v3ALsCcO098060: to=<
lemontree66 at sbcglobal.net>, delay=3+19:23:38, xdelay=00:00:00,
mailer=esmtp, pri=16918651, relay=al-ip4-mx-vip1.prodigy.net., dsn=4.0.0,
stat=Deferred
Apr 14 13:17:52 dudley sm-mta[13944]: v3ALsCci098060: to=<
markj at limerockcontractors.com>, delay=3+19:23:34, xdelay=00:00:00,
mailer=esmtp, pri=16918660, relay=limerockcontractor...ction.outlook.com.,
dsn=4.0.0, stat=Deferred
Apr 14 13:17:52 dudley sm-mta[13944]: v3AFHFRc047683: to=<Dglover at ets.org>,
delay=4+02:00:25, xdelay=00:00:00, mailer=esmtp, pri=18090413, relay=
ets-org.mail.eo.outlook.com., dsn=4.0.0, stat=Deferred
Apr 14 13:17:52 dudley sm-mta[13944]: v3AFHFRR047683: to=<
Tony at pacentengineering.com>, delay=4+02:00:28, xdelay=00:00:00,
mailer=esmtp, pri=18090435, relay=pacentengineering-...ction.outlook.com.,
dsn=4.0.0, stat=Deferred
Apr 14 13:17:52 dudley sm-mta[13944]: v3ADSbiv027178: to=<
Cynthia.Franklin at erickson.com>, ctladdr=<kate at dudley.casano.com>
(1001/100), delay=4+03:49:14, xdelay=00:00:00, mailer=esmtp, pri=18390722,
relay=erickson-com.mail....ction.outlook.com., dsn=4.0.0, stat=Deferred

These mails sit in my queue, get retried, and are constantly "Deferred".

How can I find out why that is?

System is running 10.3, with sendmail.

Thanks,
Bill Dudley
amateur sysadmin