Are you sure the activedirectory plugin is the one returning the addresses? The email-ext plugin uses the MailAddressResolver from Mailer to resolve addresses for users. So, this isn't a bug with email-ext.

Not sure which plugin exactly causes this issue.

can you enable the logger for hudson.plugins.active_directory.ActiveDirectoryMailAddressResolverImpl at the FINE level and see what the logs show for when the address is resolved?

Here is it (I switched my domain and user name with [myDomain] and [myUserName]:

May 28, 2015 4:45:15 PM FINE hudson.plugins.active_directory.ActiveDirectoryMailAddressResolverImpl
Failed to look up Active Directory for e-mail address
org.acegisecurity.userdetails.UsernameNotFoundException: No such user or group: [myDomain]_[myUserName]
at hudson.plugins.active_directory.ActiveDirectoryAuthenticationProvider.getDnOfUserOrGroup(ActiveDirectoryAuthenticationProvider.java:220)
at hudson.plugins.active_directory.ActiveDirectoryAuthenticationProvider.retrieveUser(ActiveDirectoryAuthenticationProvider.java:107)
at hudson.plugins.active_directory.AbstractActiveDirectoryAuthenticationProvider.loadUserByUsername(AbstractActiveDirectoryAuthenticationProvider.java:53)
at hudson.plugins.active_directory.ActiveDirectoryMailAddressResolverImpl.findMailAddressFor(ActiveDirectoryMailAddressResolverImpl.java:54)
at hudson.tasks.MailAddressResolver.resolve(MailAddressResolver.java:122)
at hudson.tasks.Mailer$UserProperty.getAddress(Mailer.java:560)
at hudson.plugins.tfs.model.TfsUserLookup.find(TfsUserLookup.java:32)
at hudson.plugins.tfs.model.Project.convertServerChangeset(Project.java:63)
at hudson.plugins.tfs.model.Project.getVCCHistory(Project.java:110)
at hudson.plugins.tfs.model.Project.getDetailedHistory(Project.java:130)
at hudson.plugins.tfs.actions.CheckoutAction.checkout(CheckoutAction.java:38)
at hudson.plugins.tfs.TeamFoundationServerScm.checkout(TeamFoundationServerScm.java:214)
at hudson.model.AbstractProject.checkout(AbstractProject.java:1276)
at hudson.model.AbstractBuild$AbstractBuildExecution.defaultCheckout(AbstractBuild.java:610)
at jenkins.scm.SCMCheckoutStrategy.checkout(SCMCheckoutStrategy.java:86)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:532)
at hudson.model.Run.execute(Run.java:1744)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:98)
at hudson.model.Executor.run(Executor.java:374)

How does TFS store your username? Can you add a logger for "hudson.plugins.tfs.model.TfsUserLookup" as well?

I don't know how TFS stores my username.
Adding "hudson.plugins.tfs.model.TfsUserLookup" added the following line before the ActiveDirectoryMailAddressResolverImpl log:
"
May 28, 2015 6:28:37 PM FINE hudson.plugins.tfs.model.TfsUserLookup
Looking up Jenkins user for TFS account '%s'.
"

Oh, that's fantastic...they don't have the value for the account name. What my guess is happening is that the user account name comes in as domain\username, this is passed to the MailAddressResolver, which converts the \ into a _. It doesn't find a user in the Jenkins database with that account and so it can't resolve the address correctly. Can you go to http://YOURSERVER/jenkins/asynchPeople/ and see if there are user accounts in the form of domain_user?