I always assumed it was more a sanitization issue for security's sake. By allowing only a simple subset ("common") email address type, you can be ambivalent about what email server is running and how it reacts to the wide variety of specially crafted email addresses.
With no validation other than sending the email, you have to know, for example, what the server would do with an email address that claims to be @localhost. Now it becomes a problem- or at least a question and concern- for the backend system. Whether the backend interprets root@localhost as valid and does exactly what it's told or rejects it due to some configuration- it has become a backend complication and a DOS attack vector.
A simple policy of only handling a subset- the common class of email addresses- is one of the things that allows us to have a simple mental model of what the MTA is supposed to do. The fact that it sometimes caught a type-o, or not, is incidental. "Invalid email" wasn't meant to imply the email address doesn't fit the spec- it was meant to imply that a particular site or service has chosen not to accept email addresses like that.
With no validation other than sending the email, you have to know, for example, what the server would do with an email address that claims to be @localhost. Now it becomes a problem- or at least a question and concern- for the backend system. Whether the backend interprets root@localhost as valid and does exactly what it's told or rejects it due to some configuration- it has become a backend complication and a DOS attack vector.
A simple policy of only handling a subset- the common class of email addresses- is one of the things that allows us to have a simple mental model of what the MTA is supposed to do. The fact that it sometimes caught a type-o, or not, is incidental. "Invalid email" wasn't meant to imply the email address doesn't fit the spec- it was meant to imply that a particular site or service has chosen not to accept email addresses like that.
Or at least that's what I assumed :-)