First of all - this problem occurred, from what I can see in my mail archive, around March 1st. It could be related to the 1.452 Jenkins update.

When a build fails the emails sent to individuals who broke the build is sent to the wrong address. The e-mails are sent to DomainName_UserName@DomainName.com instead of UserName@DomainName.com or even better Full.Email.Address@DomainName.com

I see the following in the logs when a build fails:

WARNING: Credential exception tying to authenticate against DomainName.com domain
org.acegisecurity.BadCredentialsException: Authentication was successful but cannot locate the user information for DomainName_UserName

So it seems that the active directory plugin is looking for the wrong username. It should look up "UserName", not "DomainName_UserName ". When I log in to Jenkins, I can see in the logs that it looks up "UserName" in AD, which is correct. I can also see that it is provided with all email addresses, full name etc. So I guess the AD plugin is working.

I believe the problem is related to the DomainName\UserName format that Windows/Active Directory is using. Our SVN server is also using AD. When I do a "svn info" on any resource, the LastChanged Author is "DomainName\UserName".

Could it be that the SVN plugin is "sanitizing" DomainName\UserName to DomainName_UserName?