For some reason Office 365 SMTP throws below error when trying to access via SMTP from other applications or services such as Gmail.

Example error in Gmail: Secured connection using TLS (recommended) O Secured connection using SSL Authentication failed. Please check your username/password. Server returned an error: “334 VXNlcm5hbWU6 334 UGFzc3dvcmQ6 535 5.7.3 Authentication unsuccessful [HE1PR1001CA0015.EURPRD10.PROD.OUTLOOK.COM), code: 535”


The error usually occur when SMTP is not enabled in Office 365 for the affected. However, we have seen this happen even when SMTP is enabled.

A simple fix is to simply untick SMTP Authentication for the account in Office 365 and then re-enable it again. Give it 10 mins for the changes to apply and your SMTP should start working again.

Also ensure you use below settings for SMTP.

SMTP Server:

Port: 587 (TLS)

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