CERN computing facilities
This is tolerated under the CERN Computing Rules, as set out in Operational Circular No. 5, as long as it is occasional, does not encroach on work time and does not overburden the electronic network. You should bear in mind that even private email correspondence can reflect upon CERN if it sent from your CERN account. You are therefore encouraged to set up a private email address for personal correspondence.
(personal use of CERN e-mail address)
The password is yours. Consider it like your toothbrush: never share it and change it regularly. There is no legitimate reason for you ever to be asked to disclose your password -- not by the CERN Computer Security Team, the CERN Service Desk, or your colleague. In fact, sharing your password is a direct violation of the CERN Computing Rules, as set out in Operational Circular No. 5. Your colleague may not be aware of this, but you should bring it to his/her attention as you politely decline.
(protected data)