If you run the mailq command and see an "Out of memory" error as in the following example, then the recipient's email server is experiencing a memory problem.
----Q-ID---- --Size-- -----Q-Time----- ------------Sender/Recipient------------
i3DILcw21033 3415 Tue Apr 13 14:21 <eliza@ninsol.com>
(Deferred: 452 4.3.1 Out of memory)
<MWalsh@cmflines.com>
You will likely see corresponding sendmail entries in your mail log file, e.g. /var/log/maillog.
Apr 13 17:49:06 gna sendmail[21965]: i3DILcw21033: to=<MWalsh@cmflines.com>, ctladdr=<eliza@ninsol.com> (106/100), delay=03:27:27, xdelay=00:00:00, mailer=esmtp, pri=393958, relay=mail2.cmflines.com. [63.208.156.193], dsn=4.0.0, stat=Deferred: 452 4.3.1 Out of memory
