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Smtp error in Business Central

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Posted on by Asif Shahzad

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Hello all,

I m trying to set up SMTP and providing an office 365 user detail of another tenant but BC is through this error.

"A call to MailKit.Net.Smtp.SmtpClient.Authenticate failed with this

message: 5.7.139 Authentication unsuccessful, SmtpClientAuthentication is disabled for the Tenant."

Please note BC is on Tenant A and SMTP user is on tenant B.

SMTP client authentication is already enabled on TENANT B.

Please help how i can fix it.

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Suresh Kulla 50,245 Super User 2025 Season 2 on at

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If you have set up Multifactor Authentication then you need to create App Password and then use that password instead of a regular password. You can set up one here

mysignins.microsoft.com/security-info

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Marco Mels on at

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Hello,

docs.microsoft.com/.../authenticated-client-smtp-submission

I did try to repro this scenario.

Tenant A = tenantA.microsoft.com, this is where BC is sitting, all users have an Exchange license in Tenant A

Tenant B = tenantB@microsoft.com, this is where Office 365 SMTP is sitting, the SMTP user is licensed in Tenant B

Users connect via Azure AD authentication to their Exchange Online in Tenant A via user1@tenanta.onmicrosoft.com

The smtp user in BC in Tenant A is as follows:

server: smtp.office365.com

port: 587

username: smtp@tenantb.onmicrosoft.com

UseSsl = True

Password = secured string

This scenario will work.

It is correct that Azure AD conditional policies or security defaults policies can break the SMTP solution in BC. MFA is supported but only on a per user base where App passwords are required.

You can verify via Send-MailMessage powershell command outside BC if mail flow works. If it does not work via PowerShell, it will also not work via BC.

Hope it helps.

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Asif Shahzad 410 on at

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Hello Marco, thank for your reply. I did same but still not working.

system is through error of MFA, though that is disabled.

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Chris Bulson on at

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The solution is in the error, SMTP authentication is disabled on the tenant. This can only be checked\fixed through Powershell.

community.dynamics.com/.../troubleshooting-smtp-mail-setup-error-5-7-3-authentication-unsuccessful-in-dynamics-365-business-central

You must confirm that both Basic Auth and SMTP are enabled per the 2 different articles and PowerShell examples linked in the above blog. The only other thing that would cause this is if you have 'Conditional Access Policies' setup in AAD. You would have to look at this through portal.azure.com to verify.

Thanks,

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