Resolving SQL Server DatabaseMail Error: Cannot send mails to mail server. (Failure sending mail.)

I learned some information I thought could help others more quickly resolve this error on SQL Server. As security demands increase, some features of older versions of products get more difficult to maintain. In this case, I am supporting a SQL Server running on SQL Server 2014. It has the latest Cumulative Update applied (CU) yet the emails sent via DatabaseMail are still failing. They were failing occasionally but began to fail consistently after applying one of those recent updates. In conjunction with the updates we applied registry entry changes in an attempt to get SQL Server to support TLS 1.2+ with Microsoft Office Email on Smtp.office365.com port 587.

The error we received from the failed emails was very vague:

The mail could not be sent to the recipients because of the mail server failure. (Sending Mail using Account 3 (2022-01-31T06:18:01). Exception Message: Cannot send mails to mail server. (Failure sending mail.).

I used this simple script to test sending emails:

declare @rc int
  exec @rc = msdb.dbo.sp_send_dbmail
      @profile_name = N'SQLSend',
      @recipients         =  'myemail@mail.com',
      @subject    = N'*** Test by me ***',
      @body     = 'email body'

I wrote a C# .Net program to see if I could get more error detail. I didn’t know what version of .Net Framework was on the server so I compiled against .Net 2.0 and ran my program and got the same error. Then I compiled my program on .Net 4.8 and got a different error. What I learned was that the newer version of the .Net framework (anything 4.0 or higher) provides a more detailed error message.

So my next task was to figure out if SQL Server is using the .Net Framework to send the Email (which I think it is) and how to tell SQL Server to use a newer version of the .Net Framework.

SQL Server uses an executable named DatabaseMail.exe to send the email. That program will exist in a folder named something like: D:\Program Files\Microsoft SQL Server\MSSQL12.MSSQLSERVER\MSSQL\Binn

There program is a .Net program and therefore includes a .Config file. Or, I should say, it “can” include a config file. I think what happened is that .config file for our DatabaseMail.exe got deleted when an SQL Server update was installed (as mentioned in this article: https://support.microsoft.com/en-us/topic/kb3186435-fix-sql-server-2016-database-mail-does-not-work-on-a-computer-that-does-not-have-the-net-framework-3-5-installed-or-stops-working-after-applying-sql-server-update-3480beb6-1329-74d6-0f3a-e8e3d893326c

So I used notepad to create a new file named databasemail.exe.config in the same folder, and I put this content into the file:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <appSettings>
    <add key="DatabaseServerName" value="." />
    <add key="DatabaseName" value="msdb" />
  </appSettings>
  <startup> 
        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
    </startup>
</configuration>

By telling DatabaseMail.exe to use .Net Framework 4.5, we were able to get a more detailed error message from the failed emails. In our case, the error we got by using .Net Framework 4.5 is shown below. Our belief is that one of the SQL Server updates somehow altered the existing configured password of the hashing or encryption algorithm used for it. We simply re-entered our password in DatabaseMail configuration and our email started working again.

The mail could not be sent to the recipients because of the mail server failure. (Sending Mail using Account 3 (2022-02-01T12:47:47). Exception Message: Cannot send mails to mail server. (The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.57 Client not authenticated to send mail. Error: 535 5.7.139 Authentication unsuccessful, the user credentials were incorrect.