Office 365 - SMTP can't authenticate

procc 16

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?

Jade Liang-MSFT 9,976 Reputation points Microsoft Employee

2020-10-26T09:11:12.95+00:00

The issue is more related to exchange online, in order to solve it better, I added the exchange online tag. Thanks for your understanding.
Imranul Bashar 6 Reputation points

2021-06-08T13:00:29.96+00:00

I am having an error syncing Gmail and Outlook. I get the error as
Authentication failed. Please check your username/password.
Server returned error: "334 VXNlcm5hbWU6 334 UGFzc3dvcmQ6 451 4.7.0 Temporary server error. Please try again later. PRX4 [AM0PR10CA0052.EURPRD10.PROD.OUTLOOK.COM] , code: 451"
Ariel Radunz 1 Reputation point

2021-07-23T18:28:57.283+00:00

MFA is enable?...

if MFA authentication is enabled, it cannot be enforced
yk 1 Reputation point

2022-05-11T13:07:06.38+00:00

I'm also having this error since loooooong time ago.
I've tried enabling 2FA, generating an app password, but still no luck.
It really is annoying.
schlagmichdoch 1 Reputation point

2022-05-24T16:32:17.897+00:00

For me this issue occured when I changed my outlook password and forgot to change it in my gmail account settings.

E-Mail reception still worked seemlessly as this was handled as a redirect by the outlook account but when sending (in my case responding) I got this very error. Editing the account information under gmail>settings>Account and Imports>Send mail as>edit info fixed it.

Foolish me. Maybe someone can benefit
FloM 1 Reputation point

2022-05-24T19:11:08.707+00:00

Hi,

smtp service need to be enabled on the mailbox (ok)
if mfa is enabled too you'll need to generate an app password (ok)
if smtp settings are good too
(cf. https://support.microsoft.com/en-us/topic/8361e398-8af4-4e97-b147-6c6c4ac95353 )
(or https://learn.microsoft.com/en-us/exchange/mail-flow-best-practices/how-to-set-up-a-multifunction-device-or-application-to-send-email-using-microsoft-365-or-office-365 )

could it be a TLS auth issue due to the TLS deprecation ? (TLS 1.0 & 1.1 deprecation in Office 365)
TLS 1.2 may be now required....

16 answers

Martin Rudloff 6 Reputation points

2021-03-05T16:06:46.61+00:00

Same issue with SMTP here while trying to get GMail sending mails using my O365 address. Tried to set it up today for the first time. Error message is:

334 VXNlcm5hbWU6 334 UGFzc3dvcmQ6 535 5.7.3 Authentication unsuccessful [AM0PR04CA0033.eurprd04.prod.outlook.com] , code: 535
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Anthony Rich 21 Reputation points

2021-04-04T15:17:10.99+00:00

same issue Authentication failed. Please check your username/password.
Server returned error: "334 VXNlcm5hbWU6 334 UGFzc3dvcmQ6 535 5.7.3 Authentication unsuccessful [BL1PR13CA0349.namprd13.prod.outlook.com] , code: 535"
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info inauguralseason.com 6 Reputation points

2021-05-10T00:08:50.667+00:00

I referenced this forum with my chat with Go daddy's chat help.

They told me:

"I could see SMTP is not enabled, that's why it is not working, after the migration, Microsoft has turned off that by default for all users.

I am enabling it for you with help of my advanced technical team, it will take 4-5 minutes.
Within 24 hours, you will be able to send/receive email with your Gmail."

So it appears to be a setting the toggled off for everyone. Just need to reach out for now.
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Richard Holmes 1 Reputation point

2021-09-01T20:34:35.673+00:00

I'm chatting to GoDaddy now and they are telling me this is a Google issue, not Microsoft.

Can you clarify if this setting was toggled off by Google or by GoDaddy to fix this issue?
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nka 21 Reputation points

2021-07-31T06:01:17.267+00:00

Any solution to this thread. MFA is now enabled by my organization and I can no longer send or receive Office 365 Outlook mail to my Gmail. Thank you.
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Ahmed 6 Reputation points

2021-11-04T15:14:51.107+00:00

By turning off 2 factor authentication in my Microsoft account, I was able to set it up as "send mail as" in my gmail account. Afterwards, I re-enabled 2FA.
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Office 365 - SMTP can't authenticate

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