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: smtp.office.365.com
Port: 587 (TLS)