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BugZero found this defect 2149 days ago.
You need to configure email notifications to be sent to a Gmail or Google Apps Mail account in Veeam ONE.
Use of port 587 is required for TLS when communicating with Gmail or Google Apps Mail.
Set the Port to 587 in Veeam ONE console > Options > Server Settings > SMTP Settings. Ensure that both Enable SSL security and Use authentication checkboxes are checked.
More information is available in this Google KB. In some rare cases the ‘Access for less secure apps’ may need to be Turned On. For details, check this KB article.