The letters of poet and playwright WB Yeats are both. It was the intimate art, perhaps. It was understood as something to be cherished. Someone took a slice of their valuable time and they gave it to you, gave you their undivided attention as they wrote that letter. One wonders how often those writers write letters anymore.
Email sometimes contains witticisms.
It can be literary, if the writer is so inclined. I know that. But email is rarely saved for long. Yes there are people who still write letters as literature, but it s getting rarer. The email form, for one thing, doesn t encourage coherent literary expression.