You need to fix CS. Also you should allow to specify separator (not hardcode ,). There is missing trim() call after explode (ie. [email protected], [email protected], [email protected]) and IMO there should be two different msgs for error (one for multiple and one for single).

I fixed the code to adhere to the coding convention, implemented the suggested trimming of the split values and added a separate error message for the case that the multiple option is set.
Regarding the separator, I wanted to be as close as possible to the RFC and it only allows a comma as a separator.

I'm not convinced to be strict to RFC because I'm still rewriting EmailValidator (see #1581) and I saw many strange valid email addresses already ;-) But of course if you think its good enough and @fabpot agree I give +1 until for merge (if I dont end refactoring and making my PR for EmailValidator).

@stloyd I thought about your comment once more and you got me convinced, so I made the separator configurable in the constraint as well (with the standard comma being the default).

The email validator is just for the addr-spec part of the specification (3.4.1), not about validating the address. As specified in section 3.4, the address is in fact richer than just what is implemented here.