Oh, Joomla... The code which tries to send the email is inside a try/catch block. JMail is supposed to throw an exception if the recipient list is invalid which is caught and suppressed. But no. JMail will suppress it and do something different instead.
As to why there is no check for an empty list: because it would not be correct. You enter the list of email addresses as a comma separated list. The following is a non-empty list:
It only has empty (hence invalid) addresses. Also, the following is a non-empty list too:
It has an empty (invalid) address in the middle. The same goes for
email@example.com,i am * obviously invalid@---not-even-a_valid_domain.com
The thing is that Admin Tools should not be doing email address verification. It's the job of the mail library to do that. The mail library should then return an error which can be caught and suppressed programmatically. This is exactly what PHPmailer does, either by returning FALSE (old versions) or throwing an exception (newer versions, feature activated since Joomla 3.5).
The problem is that JMail is a wrapper to PHPmailer which does some stupid things, like suppressing the exceptions thrown by phpMailer's intrinsic methods and returning a boolean. This is quite the opposite of what it does for Send(). So, this problem is triggered by inconsistent error handling in Joomla.
For now I recommend adding an email recipient for security exceptions email OR disabling the configuration monitoring for Global Configuration and extensions in Admin Tools. I strongly recommend the former.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!