Office 365 - SMTP can't authenticate

Question

Hi

When I try to connect to Office365 SMTP server, I get this error:

Server returned error: "334 VXNlcm5hbWU6 334 UGFzc3dvcmQ6 535 5.7.3 Authentication unsuccessful [BN6PR14CA0013.namprd14.prod.outlook.com] , code: 535"
When I try to connect to POP server, it works without a problem.
If I try to connect to SMTP server with user@ssss .onmicrosoft.com it works without a problem, but if I try to connect with user@ssss .com it gives me error.
Checked many settings in "Microsoft 365 admin center", I think everywhere SMTP is turened on, and should work.
But maybe I missed some setting?

Answer 1

I am having this same error, it just started today:
Server returned error: "334 VXNlcm5hbWU6 334 UGFzc3dvcmQ6 535 5.7.3 Authentication unsuccessful [FRYP281CA0008.DEUP281.PROD.OUTLOOK.COM] , code: 535"

I am fully able to receive emails from my outlook account on my gmail account. I just cannot send emails.

Was there any fix?

Answer 2

Judging from the glut of recent replies, a lot of people are having this issue. This post indicates that it's a security challenge coming from Microsoft when the Gmail service attempts to access through SMTP. The proposed fix is to go into the recent sign-ins section of your Office365 account and confirm those activities were you which should stop the extra security check. Unfortunately, it appears that since that solution was posted in 2014, the ability to mark a sign-in as "safe" is no longer there, instead it's just a button to indicate that something isn't safe.

If the linked post is correct about the issue, Microsoft should do something about it soon. The hassle from this error is reinforcing the dislike of Outlook that likely had people using SMTP to begin with.

Answer 3

I had this issue and I solved doing the following. You need to activate "Two-step verification" for your account. Then you create an "App password" from the Advanced Security settings. Then you need to use that generated password for the SMTP server. Took me a while to figure it out but now this works for me.

Answer 4

Ping smtp.office365.com in command prompt and note the ping report as below:

Use the smtp address shown for your location and try using that address and see if it helps

Answer 5

Hi @procc ,

If I try to connect to SMTP server with user@ssss .onmicrosoft.com it works without a problem, but if I try to connect with user@ssss .com it gives me error

Are there any differences between these 2 accounts?

Due to the issue only happen to this specific account, it may be related to the account itself. I have researched a lot about your error and found it may happen when you configure a Microsoft 365 account and don't specify the correct SMTP server in Gmail. For more information, please refer to this article: How to find the correct SMTP server settings so that you can Send Mail As from Office365 in Gmail(Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)

For your error, the ping result address may be your Office 365 SMTP server address, it's suggested to change the server address to the ping result as Manu mentioned to check if your issue has any difference.

If your issue still exists, considering you also couldn't configure it in Outlook, in order to perform more targeted test, please provide me with more information about your account and the version of your Outlook Client(File>Office account>About Outlook).

If your issue has any update, please feel free to post back.

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