I use a gmail account as the default system email, then I try to send several emails to test ticket creation, it just work properly, all received email automatically create a ticket on the system.

Then I change to use another email address of a local mail server (in other network) as the default system email. Although all of the test emails are received (using Outlook to check), Less than half of the test emails automatically create a ticket on the system.

Can anyone give me some suggestions please?