I manage a googlegroups with many hundreds of people, and often happens that spam is sent to our mailing list even if it's for members only.

The most annoying is when the spam arrives with the address of members.

Which suggestions can I give to the people to reduce the risk of their mail being harvested by spam bots? I usually suggest to enable https on their webclient and SSL on their desktop client and to run an antivirus session regularly.

Is this a good way to avoid the problem or it doesn't change anything?