Describe the bug
We encountered an incident where our mail sending utility (which uses MailKit version 3.5.0) one day started prepending the domain name to the username before then trying to perform the authentication with our SMTP server. One day the domain name was not being prepended, the next it was.

We believe there may be an environment variable or system setting or GPO policy that must have affected the MailKit to start behaving in this way. But as we do not know, we are asking for your assistance in understanding what external factors would affect the behavior of the SmtpClient.Authenticate() method.

Platform (please complete the following information):

• OS: Windows Server 2019
• .NET Runtime: Net 7.0
• .NET Framework: Net 7.0
• MailKit Version: 3.5.0

Exception
We need a root cause as to what would have made SmtpClient.Authenticate() start prepending the domain name to the username.

To Reproduce
This is in essence what we are asking of you. What would cause SmtpClient.Authenticate() to start prepending the domain name to the username (like "abcxyz\mymailuser").

Expected behavior
We would expect SmtpClient.Authenticate() to make use of the provided username without adulteration.

Code Snippets
If applicable, add code snippets to help explain your problem.

// Add your code snippet here.

Protocol Logs
Not available.

Note: if the protocol log contains sensitive information, feel free to email it to me at
jestedfa@microsoft.com instead of including it in the GitHub issue.

Additional context
Your MailKit has been a great boon to our development efforts. Looking forward to more of your good work!