SSL Cert with 2048 4096 bit

As the 1024 bit cert's will get removed from most browser's in 2010 you need to create new certs with at least 2048 bit.

Zimbra 5.0.x has however only a 1024 bit default.

To change this behaviour:
Edit /opt/zimbra/bin/zmcertmgt
Replace all 1024 occurences with 2048 bit

Save and generate CSR from server and you should see no problem.

Zimbra has announced this should be fixed in release 6.0.x