What is case sensitivity in local parts?
According to RFC 5322, the local part can be case sensitive. This means Captain@tidalmail.com and captain@tidalmail.com could be different mailboxes.
In practice no major provider enforces this. They follow Postel’s Law which encourages systems to accept variations generously and treat all local parts as case insensitive to prevent confusion. This is another moment where Postel’s Law guides smooth sailing by encouraging systems to accept variations rather than reject them.
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