@doenietzomoeilijk I deliberately did not reverse my statement 'if it arrives, it's valid' does not mean 'if it doesn't arrive it's invalid'.
But give email some slack: it can handle downtime perfect. It can handle improper implementations. 'Arrived' may imply 'it can be read', but that's secondary.
And no: if a person can read the email, the address was valid. Fully. No 'reasonably sure' there, but 100% sure.
(Again: that does not mean if a person cannot read it, its invalid.)
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