My text emails arrive as attachments

I have a 2021 M1 iMac connected to ethernet and a 2021 M1 Pro MacBook using wifi. Both run the current operating system and use Mac Mail but I've had this issue with previous operating systems. I have clients that when I email them they receive the email as an attachment. This is the entire email and not an attachment that I added to the email. So for example I'll type out an email and they have to click on an attachment to view that email. The odd thing is that it doesn't happen when I use my laptop so there either has to be a setting that is different or running through ethernet does something differently. I have tried both plain and html formats. I've attached email of what it looks like on their end. Again when I send through my laptop it arrives normal.