Note exim problems.

doc/install/installation.md

@@ -78,7 +78,7 @@ Is the system packaged Git too old? Remove it and compile from source.
     # When editing config/gitlab.yml (Step 5), change the git bin_path to /usr/local/bin/git
-**Note:** In order to receive mail notifications, make sure to install a mail server. By default, Debian is shipped with exim4 whereas Ubuntu does not ship with one. The recommended mail server is postfix and you can install it with:
+**Note:** In order to receive mail notifications, make sure to install a mail server. By default, Debian is shipped with exim4 but this [has problems](https://github.com/gitlabhq/gitlabhq/issues/4866#issuecomment-32726573) while Ubuntu does not ship with one. The recommended mail server is postfix and you can install it with:
     sudo apt-get install -y postfix