Connection to smtp.office365.com fails with error "535 5.7.3 Authentication unsuccessful" due to ServiceNow source IP addresses blocked on Office365 Issue You may encounter an issue when setting up a new email account with the following properties: Server: smtp.office365.comPort: 587Connection Security: STARTTLS Connection may fail with the error:WARNING *** WARNING *** Cannot connect to SMTP server: smtp.office365.com, as: abc@xyz.com, message: 535 5.7.3 Authentication unsuccessfulCauseThis can happen for many reasons, but the most common cause is that the ServiceNow source IP addresses are blocked on the Office365 side, which maybe configured with a conditional access rule. To find the exact cause, check the Azure logs for failed authentication and the reason for the failure should be documented there. ResolutionObtain the source IP ranges from the IP Information page from your instance. Finding the IP information for your instance The IP ranges required to be inclusion listed are listed in the second row named Source address used for integrations into customer network with NO VPN. NOTE: The fix can take a few minutes to propagate in Office365. Allow this time before testing the connection again from ServiceNow. Related LinksSee also Instance can not connect to SMTP server with message "535 5.7.3 Authentication unsuccessful" due to changes in email account or email server