Does the system explicitly support 2+ email addresses for a single participant?

This is not a bug. It only supports one email/per participant.

I don't understand how the same functionality in the GUI works and from the API doesn't. Basically it should be one function that is being called. This sounds like a bug to me, but it is your call.

I will test this in the GUI and get back to you

Does the system explicitly support 2+ email addresses for a single participant?

Yes, always did.

Indeed, it worked on the GUI.

@vladro93 Maybe you can suggest a pull request or fix if you already know the problematic place in the code?

c_schimtz, if the GUI works doesn't it imply that the system supports 2+ emails?

Also I have a hunch where the problem might be:

This gets called when using invite_participants from the API

if (preg_match($pattern, $oToken['email']) !== 1) {
unset($aResultTokens[$key]);
}

And this gets called from the GUI:
$aEmailaddresses = preg_split("/(,|;)/", $this->oToken->email);

I think that if should be the same in both scenarios.

@ollehar i can try this, but first I have to make sure that what I found is a bug and not a new feature, as in the discussion above.

Yeah, it's probably a bug. :)

Here is the pull request: https://github.com/LimeSurvey/LimeSurvey/pull/2024/files

Thank you for the PR, we will test and write to you if there are any questions.

Didn't find a way to easily send updates to the original PR, so created a new one.
https://github.com/LimeSurvey/LimeSurvey/pull/2041

I updated the indentation.

Also added 'continue' after the failed validations.
https://github.com/LimeSurvey/LimeSurvey/pull/2024/files#diff-22d09ac4b2de7e29721c3fc14eb239f215498a280b8c37f4b718074646f01f5dR2664

From what I tested it works OK.

Apart from that, If I'm not mistaken, when you send the emails from the UI, there is no validation.
There is one when adding a new token, but not when sending.

The one used for adding a new token uses a more complex mechanism, based on LimeMailer::validateAddress().
The current fix is OK as it updates the current RC validation to allow multiple address (minimalistic intervention), still, it is to highlight that it doens't use the same mechanism as in the UI addNew method.

Nevertheless, we are comparing addNew operations with sendEmail operation. Why should we validate when sending? Does that happen on UI version?

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=32771