Hi Dan,
I don't think we've seen that particular issue before.
There are similarities to a known issue in Windows 10 that affects how things are rendered. This issue only occurs on certain machines (trigger still unknown), it can be fixed by deleting and recreating the user profile (would not recommend that), but it would always resurface again after a while.
The permanent fix for that is:
- Find PSMail.exe in your installation folder
- Right-click PSMail.exe, select Properties
- In the Compatibility tab, place a checkmark next to "Override high DPI scaling behavior"
- Change that setting to System (the default value is Application)
- OK
If that doesn't help I'm not sure what else to try, please consider opening a support ticket.