Are the email files owned by the appropriate user(s)?
On my server, for example -
# cd /home/e-smith/files/users/mmccarn/Maildir
# ls -l |head -10
total 8576
drwxrwsr-x+ 2 mmccarn mmccarn 352256 Sep 5 21:52 cur
-rw-rw-r--+ 1 mmccarn mmccarn 0 Jul 2 2017 dovecot-acl-list
-rw-rw-r--+ 1 mmccarn mmccarn 32072 Sep 5 08:01 dovecot.index
-rw-rw-r--+ 1 mmccarn mmccarn 1465344 Sep 5 21:52 dovecot.index.cache
-rw-rw-r--+ 1 mmccarn mmccarn 12052 Sep 5 21:55 dovecot.index.log
-rw-rw-r--+ 1 mmccarn mmccarn 3603343 Aug 2 08:38 dovecot.index.search
-rw-rw-r--+ 1 mmccarn mmccarn 3171176 Aug 2 08:38 dovecot.index.search.uids
-rw-rw-r--+ 1 mmccarn mmccarn 169 Aug 14 2016 dovecot-keywords
-rwxrwxr-x+ 1 mmccarn mmccarn 0 Feb 2 2012 dovecot-shared
# cd /home/e-smith/files/users/mmccarn/Maildir/cur
# ls -l |head 10
-rw-rw-r--+ 1 mmccarn mmccarn 20326 Jan 1 2009 1230832511.M329997P11901.office,S=20326,W=20778:2,k
-rw-rw-r--+ 1 mmccarn mmccarn 45073 Jan 1 2009 1230833540.M983888P12019.office,S=45073,W=46251:2,k
-rw-rw-r--+ 1 mmccarn mmccarn 32991 Jan 1 2009 1230843059.M83673P13104.office,S=32991,W=33664:2,k
-rw-rw-r--+ 1 mmccarn mmccarn 32921 Jan 2 2009 1230902285.M182663P7548.office,S=32921,W=33473:2,k
-rw-rw-r--+ 1 mmccarn mmccarn 49864 Jan 11 2009 1231683708.M320305P7866.office,S=49864,W=50603:2,k
-rw-rw-r--+ 1 mmccarn mmccarn 76374 Jan 13 2009 1231855812.M516476P10047.office,S=76374,W=78025:2,k
-rw-rw-r--+ 1 mmccarn mmccarn 64981 Jan 13 2009 1231862869.M780620P11268.office,S=64981,W=66468:2,k
-rw-rw-r--+ 1 mmccarn mmccarn 10744 Jan 13 2009 1231871200.M637751P12662.office,S=10744,W=10907:2,RS
-rw-rw-r--+ 1 mmccarn mmccarn 27130 Jan 13 2009 1231893517.M544619P16331.office,S=27130,W=27722:2,S
If the ownership turns out to be wrong, this command will list the commands you could use to fix ownership for all users.
(remove "echo" to run the commands, or copy/paste output to run it manually)
for u in $(db accounts show |grep \=user |sed s/\=.*//); do echo chown -R $u:$u /home/e-smith/files/users/$u/Maildir; done