OTRS Error: Can't create PID PostMasterMailbox

[cordeos]

In case this might help others, we have found the most common reason we get the following OTRS error:
Can't create PID PostMasterMailbox, because it's already running
is because multiple CRONTAB jobs are created, usually because of tmp files in otrs/var/cron folder.

Here is how to check and fix this problem...

Assuming you are using LINUX, first run the shell command:
for user in $(cut -f1 -d: /etc/passwd); do crontab -u $user -l; done

This will show you if there are multiple users with OTRS related cron jobs. If only one user has OTRS CRONTAB jobs, then:
crontab -e -u otrs

This will open the otrs user CRONTAB for edit. Go through the lines and check if you have duplicate cron jobs or statements. If you have multiple postmaster cron jobs listed, here is how to fix them:

1) delete the entire contents of the otrs user crontab (it will be recreated by the OTRS scheduler)
2) restart crond and otrs service - then check if the otrs user crontab has been created with the duplicate job entries
3) again delete the entire contents of the otrs user crontab
4) delete any tmp files in /opt/otrs/var/cron (these usually end in "~" character, such as "")
5) check your actual setup files in /opt/otrs/var/cron dont have any accidental duplicates
6) restart crond and otrs service
7) confirm you have no more duplicate crontab job entries

[crythias]

The most common reasons are (in no particular order):
1) It's not completing in the time allotted (people setting 1 minute fetches and large attachments incoming)
2) Exchange/mail server isn't listening/stopped services/bad credentials
3) multiple cron runs.