Was this set up working before or did it never work/is this a new Splunk set up?
Since there's not much information about your set up, I'd recommend checking the usual suspects like the SMTP server's details put in Splunk. If it's localhost, won't work. If it's a proper set of details, I'd recommend checking your firewall logs to see if the connections are getting denied. You can also always check with your SMTP server team to see if the credentials work or not or do they expect you to use SSL/TLS or no security. Maybe the SMTP server might not be available. You should also check the port that you are attempting to connect to is correct or not.
Thank you,
Shiv
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