Commerce: notification emails to multiple custom recipients fails Notification emails keep failing to send if I have multiple addresses set. I've tried comma separated with and without a space after, both fail the same way.

The error in the craft.log:

Email error: Invalid address: (addAnAddress to): blah@stuff.com,things@stuff.com


The error in the commerce.log:

[Forced] Email “New Order - Admin” could not be sent for order “a713f1d”. Errors:
I've run into the same issue and reported it to Craft Commerce support at the beginning of April. It was acknowledged to be a bug: 


  "At the moment we are only able to support a single email address in
  the custom 'TO' field."


and improving it was added to their TODO list.

You can use BCC field (with email addresses separated by comma or semicolon) to send notification email to multiple recipients or you can create multiple emails (hooked up to the same template and status) - one for each custom email address.