Redmine

We use the email reply feature a lot in our company - so when somebody receives an update to an issue via email, they select reply directly to the email, write their reply, and this gets copied directly to the issue.

However since we upgraded to 3.0.3 we have an issue with this which is that for one particular user this doesn't work. When testing with others users it's fine, but when this one user tries, his email replies disappear. They show up in the sent messages of the email account we use for Redmine, but they do not copy to the issue. I cannot find anything in the logs when this is happening.

Does anyone have any ideas what could be causing this?

Environment details are:

Red Hat 4.8.2-16
  Redmine version                3.0.3.stable
  Ruby version                   1.9.3-p551 (2014-11-13) [x86_64-linux]
  Rails version                  4.2.1
  Database adapter               Mysql2