Email bounces back when the email is sent to a particular mailbox for an unknown reason.
If the email account exists and Exim shows routing to the mailbox correctly but the email sent to that mailbox still bounces it is time to check the mailbox itself. Sometimes the data directory is missing entirely due to it being manually removed or a maildir folder within the mailbox is missing.
Change directories to the mailbox data directory for example:
and then check if the required folders exist:
ls -ald tmp dir cur
All three of those folders should exist with the proper owner, group, and permissions.
[12:50:55 server root /home/cpuser/mail/mydomain.tld/localuser/]cPs# ls -ald cur new tmp
drwxr-x--x 2 cpuser cpuser 4096 Mar 18 2016 cur
drwxr-x--x 2 cpuser cpuser 4096 Mar 18 2016 new
drwxr-x--x 2 cpuser cpuser 24576 Jan 28 10:34 tmp
[12:51:01 server root /home/cpuser/mail/mydomain.tld/localuser/]cPs#
If any of those are missing it needs to be recreated. An example recreating the folder new if it was missing.
chown cpuser.cpuser new
chmod 751 new