How does case sensitivity work in email headers?

Email header field names are never case sensitive under RFC 5322. Clients treat From, FROM, and from exactly the same. The values inside those fields follow different rules. Domains are always case insensitive because the DNS system ignores case entirely. The local part before the at sign is technically case sensitive according to the specification, which means something like Captain@tidalmail.com and captain@tidalmail.com could be two different mailboxes.

Behind the scenes, no major provider implements true case sensitivity in the local part. They treat everything case insensitively to avoid user confusion. This is a real world application of Postel’s Law, which encourages systems to accept as much as possible to preserve interoperability.