I think you should ask in a Microsoft outlook forum and look at Outlooks options and preferences. Photoshop is not a file editor a file would not be put into a system clipboard even for the composite of visible layers. If a file would be copied to the clipboard I would expect it to be Photoshop native choice PSB even for small file. If paste is attaching in a Tiff file I thing would be the application handling the paste the is creating a tiff file to be attached to your e-mail
If there a reason for locking yourself into the file size limitation of Outlook when such sites as Dropbox, WeTransfer and similar apps are available?
If the client also has Photoshop have you considered Window > Layer Comps for all versions in a single file posted to one of the above sites?
No, clients just want preview in mail.
No external links or downloads. That's extra clutter on the drive.
Used to work for years and years.
Now suddenly it's TIFF's it pastes.. strange.
Well I know there is a terminal code to change the screenshot mode in osx to jpg/png/pdf/tiff but Osx makes PNG's as screenshots now so don't think that's linked to my problem.
I've asked MS forum.