A user has noticed that emails are no longer being sent nor received by their Kayako instance. For instance, the automated notifications/alerts to PagerDuty are not being delivered. Additionally, an error is returned when trying to send an email: "STOREDRV.Submission.Exception: InvalidLicenseException"
This type of behavior indicates an issue with the mail server/provider. If inbound email is not working then one needs to review the mail server used for the Email Queue. In case, outgoing emails are not working this would suggest an issue with the email server used in the SMTP settings.
The error message returned in this scenario suggests the mail server doesn't have a valid license which is leading to this problem of emails not flowing. This is a known issue with O365 that can occur when a shared mailbox exceeds the 50GB limit and requires a valid license to continue working. Refer to the O365 Shared Mailboxes article for further details on how to resolve this matter.
Once the problem with the mail server is sorted out, emails should flow without any issues.