A couple of users have hit there quote limits on there email accounts, instead of the server queue or bouncing the email back to the sender the emails are just disappearing. I only noticed due to trying to email them and talking on the phone realised they didnt get any of the emails. Below is a test I did sending an email to test@domain which has a quote limit maxed.
maillog
Jan 11 23:54:07 server1 spamd[3102]: spamd: connection from localhost [::1]:58236 to port 783, fd 6
Jan 11 23:54:07 server1 spamd[3102]: spamd: setuid to test@domain.co.uk succeeded
Jan 11 23:54:07 server1 spamd[3102]: spamd: processing message <ca209193-4566-3f63-2e2c-e8a5f2ce7c7c@domain.co.uk> for test@domain.co.uk:910
Jan 11 23:54:07 server1 spamd[3102]: Error writing to /home/domain/homes/test/.spamassassin/bayes.lock.server1.domain.co.uk.3102: Disk quota exceeded at /usr/share/perl5/vendor_perl/Mail/SpamAssassin/Locker/UnixNFSSafe.pm line 88, <GEN131> line 38.
Jan 11 23:54:07 server1 spamd[3102]: plugin: eval failed: bayes: (in learn) error closing /home/domain/homes/test/.spamassassin/bayes.lock.server1.domain.co.uk.3102: at /usr/share/perl5/vendor_perl/Mail/SpamAssassin/Locker/UnixNFSSafe.pm line 118, <GEN131> line 38.
Jan 11 23:54:07 server1 spamd[3102]: spamd: clean message (-1.0/4.5) for test@domain.co.uk:910 in 0.2 seconds, 1486 bytes.
Jan 11 23:54:07 server1 spamd[3102]: spamd: result: . -1 - ALL_TRUSTED scantime=0.2,size=1486,user=test@domain.co.uk,uid=910,required_score=4.5,rhost=localhost,raddr=::1,rport=58236,mid=<ca209193-4566-3f63-2e2c-e8a5f2ce7c7c@domain.co.uk>,autolearn=unavailable autolearn_force=no
procmail
Time:1515718077 From:root@server1.domain.co.uk To:root@server1.domain.co.uk User:support-domain.co.uk Size:1303 Dest: Mode:None
procmail: Quota exceeded while writing "/home/domain/homes/test/Maildir/tmp/1515718274.18927_0.server1.domain.co.uk"
From shane@domain.co.uk Fri Jan 12 00:51:07 2018
Subject: test
Folder: **Bounced** 1660
Time:1515718274 From:shane@domain.co.uk To:test@domain.co.uk User:test-domain.co.uk Size:1715 Dest: Mode:None
procmail: Quota exceeded while writing "/home/domain/homes/test/Maildir/tmp/1515718341.19779_0.server1.domain.co.uk"
From shane@domain.co.uk Fri Jan 12 00:52:18 2018
Subject: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aa a a a a
Folder: **Bounced** 1711
Time:1515718341 From:shane@domain.co.uk To:test@domain.co.uk User:test-domain.co.uk Size:1766 Dest: Mode:None
From test@domain.co.uk Fri Jan 12 00:53:53 2018
Subject: test
Folder: /home/domain/homes/shane/Maildir/new/1515718444.20588_0 1114
Time:1515718444 From:test@domain.co.uk To:shane@domain.co.uk User:shane-domain.co.uk Size:1168 Dest:/home/domain/homes/shane/Maildir/new/1515718444.20588_0.server1.domain.co.uk Mode:None
From shane.rutter2+caf_=shane=domain.co.uk@gmail.com Fri Jan 12 00:54:33 2018
Subject: test
Folder: /home/domain/homes/shane/Maildir/new/1515718494.20753_0 6058
Time:1515718494 From:shane@domain.co.uk To:shane.rutter2@gmail.com User:shane-domain.co.uk Size:6142 Dest:/home/domain/homes/shane/Maildir/new/1515718494.20753_0.server1.domain.co.uk Mode:None
procmail: Quota exceeded while writing "/home/domain/homes/test/Maildir/tmp/1515718554.21503_0.server1.domain.co.uk"
From shane@domain.co.uk Fri Jan 12 00:55:51 2018
Subject: Re: test
Folder: **Bounced** 1856
Time:1515718554 From:shane@domain.co.uk To:test@domain.co.uk User:test-domain.co.uk Size:1911 Dest: Mode:None