Trying to set up alerting in Grafana. It worked for a few months and then suddenly it stopped sending alerts to the mail. When I tried to send a test notification to the mail, found errors logged in a log file.
logger=alerting.notifier.email t=2023-01-17T15:20:24.939973555+05:30 level=error msg=“Missing receiver”
logger=alerting.notifier.email t=2023-01-17T15:20:24.940075398+05:30 level=error msg=“Missing group labels”
Same here with a fresh Grafana 9.4.1 setup!
When testing the the default mail contact the UI says success but no mail and in the logs I also get:
logger=ngalert.notifier.email t=2023-02-28T14:28:55.8849611Z level=error msg=“Missing receiver”
logger=ngalert.notifier.email t=2023-02-28T14:28:55.8849611Z level=error msg=“Missing group labels”
The alerting got quite complicated since v9 
Ok found out the mails have been in MS quarantine. These error messages are only informational it seems. Maybe change them to log level warning would be good.