If an email sent to the helpdesk doesn't create a ticket:
1. If you use POP/IMAP/O365 (i.e. not out provided SaaS mailbox) and you're sure that the email reaches the support mailbox - check that the helpdesk app deletes those emails. Or marks them as "read". This is a sign that the email was collected. If the emails are still there - it means there was an error connecting or pulling emails. Click the "Test" button in "Admin - Email - Incoming mailboxes" to do a test run and see if there are any errors.
2. Make sure that the sender has a user account with your helpdesk or that your helpdesk accepts emails from unregistered users. And if the user is present - make sure the user is ENABLED, otherwise his emails would be dropped and won't create any tickets.
3. Make sure email notifications are enabled both globally (Administration > Email settings) and for the user's profile.
4. Make sure the email is not sent by a robot (e.g. mailer-daemon, postmaster, etc.), and it's Subject line does not contain delivery status, out-of-office notifications and other common autoresponder terms; the helpdesk program filters those out, i.e. deletes the messages without creating tickets from them.
5. Review the audit log under "Reports - Audit log". If a message was rejected by the system, there should be a message there.
P.S. You can disable the "useless email filter". In recent versions this is controlled under "Admin - Email - Disabled useless filter for specific domains". For older self-hosted installations this setting is in the appsettings.json file. To disable the filtering edit the file and set:
"disableUselessMessageFilter": true,
In very old versions for .NET Framework add this line to the web.config file inside the "appSettings" section.
<add key="disableUselessMessageFilter" value="true" />