What is the antonym (opposite) of “email” to refer clearly to "traditional mail" and not to email?
What about snail mail?
According to the Cambridge Dictionary
snail mail [informal humorous]
letters or messages that are not sent by email, but in the post
As pointed by Tᴚoɯɐuo, notice that this term is informal and mildly derogatory.
The Wikipedia entry for Mail has a few possible terms you could consider.
As an AmE speaker, I use the term postal mail to clarify that I'm speaking about physical letters and not electronic messages.