Outlook inserts a winmail.dat file in virtually every outgoing message. Only other Outlook clients do anything with it.Not true. Gmail can extract it. Recent versions of Thunderbird can extract it.
I suggest stripping these so they do not continue to proliferate as useless attachments.They are not useless attachments. It turns out that when Outlook is set to RTF, *any* attachment is made into a winmail.dat. So if you delete them, then you are deleting valid attachments that are a part of the message.
Fixing it is tedious. This can be fixed for *new* recipients by setting Outlook to not use RTF. However, for any contact you have sent a message using RTF, it remembers and continues to send them as RTF with attachments as winmail.dat files. It is possible to edit the setting for each contact to stop it using RTF. Messages sent to those people no longer get the attachments inserted as normal attachments.
Deleting winmail.dat files is not the answer. Educate your users how to change the setting for the group e-mail address if you want to stop winmail.dats ending up in your group.