Of course that capability is not a native InDesign feature.
Perhaps you know something about the .LETTER file extension that I don't, but I can't imagine why any application would support the saving of its files in Undeliverable Mail format.
Maybe if you elaborate a bit more on your objectives...
I have no idea what that is, but the answer is likely to be no.
Thanks a lot for your answers!
Yes, that's what I also thought – cause I am using indesign for a longer time. And the client told me, that he needs an indesign file with the suffixe .letter.
And I searched a lot for this letter-format.. It should be in the end something like a poster which will be sent by email.. And they don't use newsletter templates.
Hope I could help you.
Thanks for your replies!