Email typos and letter case

• Addresses are typed by hand, often on a phone, with no confirmation step.
• Phones and forms capitalize the first letter automatically.
• Neighboring keys are hit by mistake, so a period becomes a comma or two letters swap places.

• Messages bounce, or go to a stranger whose address happens to match the typo.
• The same person appears as two contacts because the addresses differ only by case.
• Email is often the key used to match records, so one wrong character breaks the match.

• Check each value against the basic shape: one at sign, no spaces, and at least one period in the domain.
• List domains by frequency. Rare domains that are one letter away from a common one deserve a look.
• Count distinct values before and after lowercasing.

• Trim spaces and lowercase the domain. Domains are not case sensitive.
• Decide whether to lowercase the part before the at sign too. The standard allows it to be case sensitive, though mail systems commonly ignore case there, so lowercasing is a widely used convention for matching.
• Repair only characters that are certain to be wrong. Treat a suspected misspelling as a suggestion for a person to confirm.
• Remember that a well-formed address is not proof that the mailbox exists. Only a reply or a confirmation from the person shows that.

• Stray whitespace and invisible charactersTwo values look identical on screen but differ by a trailing space, a non-breaking space, or a hidden character.
• Near-duplicate recordsTwo rows describe the same real thing but differ enough that an exact comparison misses them.
• Inconsistent capitalizationThe same value appears in capitals, in lowercase, and in mixed case, and software treats each as different.

TermsValidation, Normalization, Identifier, Human review