If you have Thunderbird installed and is using LDAP to authorize user on Linux/Ubuntu you might get the same problem as I used to have.

The symptom could be Thunderbird crash after a minute or two.
You might see this is your /var/log/auth.log:

Jun  1 10:26:50 <host> thunderbird: nss_ldap: failed to bind to LDAP
server ldap://192.168.0.4: Can't connect to the LDAP server
Jun  1 10:26:50 <host> thunderbird: nss_ldap: could not search LDAP 
server - DSA is unavailable

I solved the problem by installeing LDAP connection daemon – nslcd

sudo apt-get install nslcd

Configure nlscd to connect to your LDAP server.

You still have to configure Thunderbird if you want to use LDAP to lookup users.