The em dash (—) is used to indicate a sudden break in thought (“I was thinking about writing a—what time did you say the movie started?”), a parenthetical statement that deserves more attention than parentheses indicate, or instead of a colon or semicolon to link clauses.
Using email for login seems a compelling choice, because an email address is guaranteed to be globally unique -- no two people will have the same email. Unfortunately, there are some problems.
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