>But given that an email address is only useful if it is valid, you need to at very least use a regex validation (this is true even for the 99% that only have one address.)
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Only one caveat - regex on e-mail addresses - "thar be dragons!" - The rules around valid e-mail addresses are really complex. You want to err on the side of just basic sanity checking (has some text before the @, some text after the @ that includes a period); anything more probably won't work and you'll "fail" a perfectly valid e-mail address.