Found this in the log:
"ERROR:root:(501, b'5.1.7 Bad sender address syntax', 'SplunkHost@sh-i-0c796565e382fd186') while sending mail"
In settings we had the "Send emails as" set to "XXXXX YYYYY" with a space. This worked fine for the past 18 months until our Splunk instance was upgraded including Python. Our emails began failing. Modified the "Send emails as" to "XXXXXYYYYY" without the space. Seems to be working again.
In settings we had the "Send emails as" set to "XXXXX YYYYY" with a space. This worked fine for the past 18 months until our Splunk instance was upgraded including Python. Our emails began failing. Modified the "Send emails as" to "XXXXXYYYYY" without the space. Seems to be working again.
If the issue is not mentioned in the release notes, consider filing a bug report.
Check Settings->Server settings->Email settings. If you still have trouble, open a support request.