What is case sensitivity in local parts?
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Technically, yes. RFC 5322 (the standard that defines email address syntax) says the local part (the bit before the @) can be case sensitive. That means Captain@tidalmail.com and captain@tidalmail.com could theoretically point to different mailboxes.
So In practice? No major provider enforces this. Gmail, Outlook, Yahoo Mail, and every other inbox you've heard of treat email addresses as case insensitive. They follow Postel's Law (be liberal in what you accept) to avoid confusing users. If someone types their address with random capitals at signup, it still works.
Why it matters for senders: you don't need to worry about case when validating addresses or matching them in your database. Normalize everything to lowercase for storage and comparison, and you'll never have a problem. If you're building a custom mail server, follow the crowd and treat local parts as case insensitive. Going strict-RFC-compliant here just creates user frustration for no benefit.
The one edge case: some legacy corporate mail systems or custom SMTP servers might enforce case sensitivity. If you're sending B2B and a delivery fails with a "user unknown" bounce, try the exact case they provided at signup. It's rare, but it happens. Bottom line: the spec says case matters, real life says it doesn't. Store addresses in lowercase, send to whatever case your subscriber gave you, and you're covered. For more on what makes an address valid (or not), check out what counts as an invalid address.
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