Emails not working on projects with non LDAP users as members

Redmine 3.2.0.stable.15058
Ubuntu 14.04

I have a redmine server that is working really well. The only problem is, none of the emails are being sent in certain projects. The only thing that those projects have in common is that they have members who are from outside of my company. Everyone inside my company uses LDAP, and these outside people use the standard login. It stops emails being sent for all people involved in the project, not just the outside people, but only for the projects that the outsiders are involved. Any idea on how to fix this?

Just for clarity, the person who isn't on my company domain, doesn't have to be involved in the ticket, just added to a project, and it breaks all emails for that project.