> But if you are concerned about subscribing to something twice
I'm concerned about some service collecting my email address and "accidentally" exposing it to spammers.
> Of more interest to me, omitted from the presentation--as almost always--is anything about what is disliked about a malformed address. You see this when some web form says it doesn't like your address, but won't say why, leaving you to guess and try things until it is satisfied.
That is indeed yet another reason why you should never ever try to parse meaning from email addresses you do not own.
And the extent of that parsing should be in accordance with the relevant RFCs (namely, 5322). Per that RFC, the local-part is an opaque string of permitted characters. Attempting to parse the local-part beyond that when you ain't the one who owns/controls that address is bug-prone at best and user-hostile at worst.
I'm concerned about some service collecting my email address and "accidentally" exposing it to spammers.
> Of more interest to me, omitted from the presentation--as almost always--is anything about what is disliked about a malformed address. You see this when some web form says it doesn't like your address, but won't say why, leaving you to guess and try things until it is satisfied.
That is indeed yet another reason why you should never ever try to parse meaning from email addresses you do not own.