Certain eZ Publish e-mails produce "undisclosed-recipients:;" in the "To" field. This causes Sendmail errors which, depending on your mail settings, will either cause error e-mails to be sent out, or delay or omit the e-mails from being sent altogether.

The error is:
"undisclosed-recipients:;... List:; syntax illegal for recipient addresses"

An example eZ Publish e-mail is the notification e-mail. Simply subscribe to notifications on a folder, edit an object in that folder, and then trigger the "frequent" cronjob.

This behavior has been reported before on EZP-13617. There, a solution is provided, in the form of an additional [MailSettings] setting:

[MailSettings]
Transport=sendmail
...
SendmailOptions[]=-O NoRecipientAction\=add-to-undisclosed

However, this no longer works.

Steps to reproduce:

1. Admin interface: create the following content structure:

Home
	Tests (folder)
		Article 1 (article)

2. Admin interface: go to Dashboard/My notification settings/ and add "Tests" folder to "My item notifications";
3. Admin interface: edit Article 1, and publish;
4. Run "runcronjobs.php frequent" script:

$ php ezpublish/console ezpublish:legacy:script runcronjobs.php frequent
Running script 'runcronjobs.php' in eZ Publish legacy context
Using siteaccess  for cronjob
Running cronjob part 'frequent'
Running cronjobs/notification.php at: 08/04/2015 5:48 pm
Starting notification event processing
undisclosed-recipients:;... List:; syntax illegal for recipient addresses			<===
Done
Completing cronjobs/notification.php at: 08/04/2015 5:49 pm
Elapsed time: 00:00:14
...

Additional steps:

5. Modify /ezpublish_legacy/settings/override/site.ini.append.php:

[MailSettings]
Transport=sendmail
...
SendmailOptions[]=-O NoRecipientAction\=add-to-undisclosed

...and clear cache;
6. Admin interface: edit Article 1, and publish;
7. Run "runcronjobs.php frequent" script:

$ php ezpublish/console ezpublish:legacy:script runcronjobs.php frequent
Running script 'runcronjobs.php' in eZ Publish legacy context
Using siteaccess  for cronjob
Running cronjob part 'frequent'
Running cronjobs/notification.php at: 08/04/2015 5:53 pm
Starting notification event processing
readcf: unknown option name NoRecipientAction\							<===
Done
Completing cronjobs/notification.php at: 08/04/2015 5:53 pm
Elapsed time: 00:00:00
...