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

Ashley White 0 Reputation points

2024-03-05T22:21:45.4033333+00:00

AFTER HOURS OF SEARCHING AND FAILURES, I FOUND THE ANSWER ACCIDENTALLY LOL

All you need to do it to go to settings (https://admin.exchange.microsoft.com/#/settings) and UNCHECK the "Turn off SMTP AUTH protocol for your organization" because it is checked by default. It worked immediately after doing this (facepalm)

I hope this helps.
Please sign in to rate this answer.

0 comments No comments
Sign in to comment

Use comments to ask for clarification, additional information, or improvements to the question.

Share via

Office 365 - SMTP can't authenticate

16 answers

Your answer