You mean in TFS or in Jenkins? If you view one of the people, does the account have a valid email address specified? On a side note, I submitted a PR to fix the logging in the TFS plugin.

Adding a logger for hudson.tasks.MailAddressResolver would be interesting too.

I mean the user IDs on link you sent me (not sure if this means Jenkins or TFS).
If I check the configuration for specific user, it shows the email with the "domain_" prefix.

May 29, 2015 12:23:36 PM FINE hudson.tasks.MailAddressResolver
Resolving e-mail address for "domain\user" ID=domain_user

If you delete the email for one of the users, it would help to see what is setting the email addresses to the domain_user@foo.com (along with adding the logger for hudson.tasks.MailAddressResolver).

Deleted, all logs and behaviors remained the same.

Can you delete the user itself from Jenkins?

I'm trying to get the MailAddressResolver to get to this point: https://github.com/jenkinsci/mailer-plugin/blob/master/src/main/java/hudson/tasks/MailAddressResolver.java#L120

Which should tell which MailAddressResolver is resolving the address.

Deleted, still the same...

May 31, 2015 11:52:50 AM FINE hudson.plugins.tfs.model.TfsUserLookup
Looking up Jenkins user for TFS account '%s'.
May 31, 2015 11:52:50 AM FINE hudson.tasks.MailAddressResolver
Resolving e-mail address for "domain_user" ID=domain_user
May 31, 2015 11:52:50 AM 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: domain_user
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)

If you are willing, could you try the following TFS plugin that fixes the logging to output the actual username? I'm interested in seeing where the username is getting changed to domain_username.

https://jenkins.ci.cloudbees.com/job/plugins/job/tfs-plugin/99/org.jenkins-ci.plugins$tfs/artifact/org.jenkins-ci.plugins/tfs/3.2.1-SNAPSHOT/tfs-3.2.1-SNAPSHOT.hpi

Not enough information to debug the issue.