How does case sensitivity work in email headers?

Email header field names (like "From", "To", "Subject") are never case sensitive. The spec is RFC 5322, and it treats From, FROM, and from exactly the same. Every email client honors this. You can't break email by capitalizing a header name differently.

The values inside those headers follow different rules. Domain names are always case insensitive because DNS itself ignores case. captain@tidalmail.com and captain@TIDALMAIL.com are the same address. The local part (the bit before the @ sign) is technically case sensitive according to the spec. Captain@tidalmail.com and captain@tidalmail.com could theoretically be two different mailboxes.

In practice, no major provider actually implements case sensitive local parts. Gmail, Outlook, Yahoo Mail, and everyone else treat them case insensitively. They do this to avoid user confusion (imagine trying to explain to a customer why captain@ and Captain@ are different mailboxes). This is a real world application of Postel's Law: be strict in what you send, liberal in what you accept.

The one place case sensitivity can bite you is custom mail servers. If you're running your own SMTP server with a config that enforces case sensitive local parts, you can create addresses that break user expectations. Don't do this unless you have a very specific technical reason and you're prepared to support the resulting confusion.

Worth knowing: some older systems (mostly Unix-based mail servers from the 1990s) did implement case sensitive local parts. You might still encounter one in a corporate environment or a university. If you're sending to an address that looks case sensitive and bounces keep happening, try the exact capitalization the recipient gave you. But this is rare enough that it's not worth worrying about unless you hit it.