[otrs] OTRS combines email with subject issue

HI, OTRS combines the email with subject ‘Advertising your message which you get - gggggggggg)’and ‘Advertising your message which you get - ttttttttttttttttttt’ and ‘Advertising your message which you get - iiiiiiiiiiiiiiiiiiiiiii’ into one ticket. These mails should be send to three tickets . Please help to study how to avoid this? What we expect is the ticket combine when subjects are same.How to configure the OTRS correctly. Thanks Frank