Failure Sending Mail - Possible Fixes

Good morning,

We have been made aware of a change in the way Microsoft Office365 handles mail, this seems to be causing a considerable number of users using their services issues with sending mail.

Office365 has changed the way it handles TLS 1.2 and on testing development have observed that when using the app password and some extra debug code to "force TLS" this worked, without the code it worked sometimes but not always. 

An example of the error that could appear is below.

Suggested solutions below: 

Firstly please ensure all systems using TMv2 meet the minimum requirements and ensure .NET4.8 (latest version) is installed, this can be found here: https://support.microsoft.com/en-us/topic/microsoft-net-framework-4-8-offline-installer-for-windows-9d23f658-3b97-68ab-d013-aa3c3e7495e0

1) As a work around use an APP PASSWORD which can be created by -

 - login into the relevant o365 user account --> clicking on user icon in upper right corner of browser (office account, not browser profile) -> view account -> my sign-ins -> security info -> add method -> app password.

When setting up the SMTP user account in TM user the email address +app password instead of the normal user password connected to MFA.

(https://docs.microsoft.com/en-us/answers/questions/512954/535-57139-authentication-unsuccessful.html)

2) Potentially looking into this which is a default setting that may need changed on your setup

 https://docs.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/authenticated-client-smtp-submission

3) If all else fails users have had good success with below however some have refused this as they see it as an unnecessary security risk despite there not being any known exploits currently, Tero Marine cannot advise on your company’s policy regarding this, but it has resulted in clearing the issue for the time being: from M365 Administration page, open Exchange Admin console.

Then Click Settings, Mail Flow, and check the box to “Turn on use of legacy TLS clients”. Click Save.

An article on how to configure legacy TLS clients for future support:

Opt into the Exchange Online endpoint for legacy TLS clients using SMTP AUTH | Microsoft Docs 

[https://docs.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/opt-in-exchange-online-endpoint-for-legacy-tls-using-smtp-auth]