> I installed SimpleMail 0.35 for 68K CPU as it was suggested

Actually, Korni, Golem, jPV and Henes suggested to install the MorphOS native version, as would be logical for use on MorphOS. Not sure why Fab recommended the m68k version.

> OS should detect that you are about to run an OS4 program and
> warn you that there are better alternatives

How should the OS know about alternatives?

> or at least tell you what are you doing

Isn't that what you told it does? ("it says "OS4 Emulator Not Installed". It gives me the link.")