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)

4 thoughts on “Solution: 535 5.7.3 authentication unsuccessful office 365 SMTP Error”
  1. I cannot find how to get to this check list, I’ve been going in circles. could you please post how to get to this specific settings. I’m not able to add “send as” account to my gmail 🙁

Leave a Reply

Your email address will not be published. Required fields are marked *