What headers are preserved when replying?

A reply preserves the logical relationship to the original through In Reply To and References. The Reply To address is also respected if the original sender set one, which can direct responses to a different mailbox such as firstmate@tidalmail.com instead of captain@tidalmail.com.

Transport related headers from the original, such as Received lines, authentication results, and Return Path, are not reused. The reply travels as a new voyage with new stamps from every harbor. Only the thread linking headers and addresses are carried forward as lineage.