From April 1, Swiss Internet Service Providers (ISPs) will have to keep a log for six months of all the emails sent by their customers.
The move has been criticised by experts who say it will be both difficult and costly to implement.
Critics also argue that loopholes in the legislation will allow criminals to slip the net.
ISPs have had over a year to comply with a change in the law under which they will now log customer activity, such as connection times, emails sent, addressees and senders.
The authorities will be able to access the stored data but only as part of an ongoing investigation into suspected criminal activities.
However, the contents of emails will not be stored. Investigators will have to make a specific request to read Internet users’ private correspondence.
