The Frequent Contacts are a special address book. It is used to manage all lately used e-mail addresses. 

Define Options

Under Frequent Contacts in the menu File -> Properties you can find Options. Here you can define which entries are archived.

Automatic Storage on / off: You can also switch the storage off completely via this function.

The storage of contacts of sent messages is the recommended setting. It may also be helpful to include the removal of addresses that have not been used for a longer period of time.

In case you do not wish to use this function, we suggest you occasionally reassess your address book, move required contacts to other address books and delete those that have become unnecessary.