hi Kevin, thanks for your help!
inside ostickets we dont use any blacklist or filtering, and if it would be filtered before, osticket would not moved the mail to the processed-folder.
i cannot find the missing messages by the message-id of the e-mail-source-code. i also searched in the headers-column, but there is no entry found. i tried the id's of two missing mails. this means it's nowhere in the system, right? but it was processed.
i copied now the original mail-file from the processed folder to the inbox that osticket uses (with linux, so nothing was changed of the headers), and now the mail did appear in the ticket and can be found also in the database of course. this is weird.
it gives me a big headdache that ostickets did not handle all mails reliable. anything i can investigate to get closer to the problem? any other ideas?