In QRadar 7.4.0, a new email server update was added to support Secure SMTP. There isn't much to go on here in this post, but I would review your configuration in the UI and look at /var/log/qradar.log for SMTP messages.

You could confirm that your email server is configured properly. Or, you might want to collect log files and open a case if you do not see any issues. I normally grep for EmailDestination when I'm looking at logs, but a support rep could also do this for you in your case.
The types of messages you would want to look for on the Console: [type=com.eventgnosis.system.ThreadedEventTerminator][parent=<ConsoleIP>:ecs-ep/EP/EmailDestination]] com.q1labs.sem.util.EmailSender: [ERROR] [NOT:0000003000][127.0.0.1/- -] [-/- -]Exception attempting to send email: Sending the email to the following server failed : localhost:25
I'm not aware of any email server issues in your specific QRadar version, but this probably needs a case to do an investigation if your configuration in the UI is correct. We have seen issues where customers with an All-in-One appliances where they add Event Collectors that the SMTP settings did not get replicated over properly. Or it could also be to an issue where the SMBTail protocol could bind up all available ports and prevent the EmailDestination from being able to send the message. This was an issue that we were not able to reproduce the SMBTail problems in our lab. So, I think having a case created to confirm what is in the logs will help. You could try to restart ecs-ep as described in IJ16965, but the logs will tell us more and we'd be able to give more advice.
https://www.ibm.com/community/qradar/home/apars/?IJ16965
Let us know if you have follow-up questions.
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