Using the latest EE 2.7.3 and MX Notify 2.4.6 (tested on a fresh install), setting up a notification for anything, eg. a new entry posted, and notifying a user group or mailing list (or combination) with more than 1 email address in the array only sends to the first email address the system processes.
I've played around with setting different email config settings, but to no avail. The system can send via the communicate tab to more than 1 address, so it must be a problem with MX Notify.
The eec.ms forums don't allow registration, so I'm posting here in case someone can help.