@berkes if it doesn't arrive, it can still be valid. Your e-mail could end up being junked, or the receiving mail server doesn't implement the protocol properly, or it's just having a really bad day, you know.
Of course, it it does end up in a mail box, you can be reasonably sure that the address is valid. But only reasonably. Not fully. Never fully.
@doenietzomoeilijk and 'valid' has an expiration. An address may be valid today, but invalid tomorrow. E.g when a person leaves a company, school, provider.
Which, again, is easily solved by not regex-validating, but just sending the mail: as long as it arrives, it is valid. As long as it is read, that's confirmed valid.
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