You face the issue, that e-mails are missing in OTRS. There could be multiple issues behind it. You want to understand how you can debug or trace the root cause of missing e-mails?
Trace and debug e-mail issues in OTRS
First of all you should recheck if you have made changes to your mail account on provider side and of course on the system on which your OTRS is working. If this is checked, you may try to fetch the e-mails once more with the debug option from your OTRS installation folder.
./bin/otrs.Console.pl Maint::PostMaster::MailAccountFetch --debug
With the default installation of OTRS on centOS:
cd /opt/otrs sudo -u otrs ./bin/otrs.Console.pl Maint::PostMaster::MailAccountFetch --debug
Check the output if you find something strange there.
Here you will find some common issues, which probably will already fix your issue.
E-Mails are too big
Your e-mail provider accepts bigger e-mails as your OTRS configuration allows. Read Configure max fetching e-mail size in OTRS to change the size in OTRS.
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