Instance can not connect to SMTP server with message "535 5.7.3 Authentication unsuccessful" due to changes in email account or email serverSummaryThe error below can be seen when performing the "test connection" in the SMTP record on the instance: "Email sender connection invalid.: Cannot connect to SMTP server: {mailserver ipaddres}, as: {user@mailserver}, message: 535 5.7.3 Authentication unsuccessful"This can be due to a change to the SMTP record configuration, or to a change on the Email Server used.ReleaseAllInstructions1 - Verify the (active) SMTP and POP3 records in the instance: /nav_to.do?uri=sys_email_account_list.do 2 - Verify using "test connection" in the SMTP record, if the same error message is logged: "Email sender connection invalid.: Cannot connect to SMTP server: {mailserver ipaddres}, as: {user@mailserver}, message: 535 5.7.3 Authentication unsuccessful". If the error occurs, contact the Email administrators associated to the SMTP Server address mentioned in the error string and ask them to verify the User credentials for the {user@mailserver} account in question. 3 - If the User account (username or password) for the SMTP server record is changed, it is important to verify if a POP3 account exists for the same user account on the Same email Server. The reason for this is that this account is likely using the same credentials. Should the credentials be changed, these would need to be updated on both SMTP and POP3 records associated to the same user@mailserver on the same Mail Service. Failing to verify this could result in fixing one while breaking the other. Related LinksChanging of the external accounts are in the scope of the engineers associated to the External mail server.The implementation of the new credentials in the ServiceNow instance are within the scope of the ServiceNow administrators associated to the customer instance. After credentials have been changed on the email accounts, it should be verified that both email sending and email receiving are tested and working as expected. See also Connection to smtp.office365.com fails with error "535 5.7.3 Authentication unsuccessful" due to ServiceNow source IP addresses blocked on Office365.