Email: legitimate duplicate email header keys with mutually exclusive values

Question

In email the Received: header can legitimately occur more than once with mutually exclusive values...

Received: three.example.com

Received: two.example.com

Received: one.example.com

Are there any other headers can occur in email headers multiple times legitimately?

Answer 1

Ya there are other headers that can appear more than once and is clearly explained here in RFC 5322

Headers that can appear more than once

• comments
• keywords
• optional-field
• trace
• resent-date
• resent-from
• resent-sender
• resent-to
• resent-cc
• resent-bcc
• resent-msg-id

Check the link above for more clarity.