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

Lydia Zhou - MSFT 2,376 Reputation points Microsoft Employee

2020-10-27T06:47:10.117+00:00

@procc

Does user@ssss .onmicrosoft.com still work when you use Outlook to setup POP account?
Do other users have the same issue when they setup POP account with Outlook client side?
Are there any modification in your organization before this issue?

Please check if you have created any authentication policy to block basic authentication for Authenticated SMTP. If so, you have to use email clients and apps supporting modern authentication to setup POP/IMAP account. Please check this for more details: Disable Basic authentication in Exchange Online.

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Lydia Zhou - MSFT 2,376 Reputation points Microsoft Employee

2020-10-30T08:03:08.353+00:00

Is there any update on this thread?
If you have any questions or need further help on this issue, please feel free to post back.

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Lydia Zhou - MSFT 2,376 Reputation points Microsoft Employee

2020-11-03T09:22:42.703+00:00

Just checking in to see if above information was helpful.
Please let us know if you would like further assistance.

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Kennedy, William 6 Reputation points

2021-01-22T03:31:55.903+00:00

any luck with this?
i am also having the same error. i tried the ping thing but that didnt work
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Edmundas 1 Reputation point

2021-01-30T14:09:19.477+00:00

No luck with this, not even Microsoft admins could tell me what is the problem... :|
They only suggest to use IMAP or etc...
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Jeff 16 Reputation points

2021-02-13T22:04:50.687+00:00

Same problem when trying to setup "Send Mail As" alias in Gmail. Error is: Authentication failed. Please check your username/password. Server returned error: "334 VXNlcm5hbWU6 334 UGFzc3dvcmQ6 535 5.7.3 Authentication unsuccessful [MN2PR13CA0006.namprd13.prod.outlook.com] , code: 535"

It previously worked for 2 years+ but stopped working last week. I have tried the CMD ping instructions and this doesn't work either.
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Charlotte Dune 6 Reputation points

2021-03-02T18:22:36.907+00:00

Mine is also not working. I can recieve, but I can't send emails. I've tried everything to make it work and it is very frustrating to find it broken.
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Ariel Radunz 1 Reputation point

2021-07-23T18:29:30.093+00:00

MFA is enable?...

if MFA authentication is enabled, it cannot be enforced
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Andrii Maslov 31 Reputation points

2021-03-03T21:15:22.217+00:00

Got the same error for SMTP, but IMAP on the other hand returns "BAD User is authenticated but not connected". Started recently. Options for IMAP/SMTP are turned on in o365 admin center.
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Office 365 - SMTP can't authenticate

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