Disabling email address suggestion, harvesting or autocompletion

Many modern email clients are designed to assist the user by automatically storing email addresses you type in, or perhaps addresses of anyone who emails you, such that when you compose a new message it can suggest address back to you when you start typing in a To or CC field.

Whilst this can be helpful it can also lead to problems, e.g. it has stored an address you typed in incorrectly and now it keeps suggesting it to you leading to further failed emails; it suggests an address/name that you think is correct but turns out not to be the intended recipient such that you risk sending email to the wrong person. In the latter case, depending on the content of the email sent, this could result in a reportable data breach.

Mail clients generally have settings that allow you to modify this default behaviour. This can help reduce the risk of mistakes, but may come at the cost of less efficient working processes (e.g. you may have to manually add extra address to an address book in the mail client).

Outlook

To disable the email address suggestion feature:

• Click the File menu and select Options.
• In the Outlook Options window click the Contacts tab
• Uncheck the 'Automatically create Outlook contacts for receipents that do not belong to an outlook address book' box
• Click ok

To remove entries from the AutoComplete cache one entry at a time, or to clear the list entirely, follow the steps in this microsoft support document (steps summarised below too)

To remove entries from the AutoComplete cache one entry at a time:

• Open a new email message.
• Type the first few characters of the AutoComplete entry that you want to remove.
• When the entry appears in the list of suggested names, move your mouse pointer over the name until it becomes highlighted, but do not click the name.
• When the "X" icon appears next to the highlighted name, click X to remove the name from the list, or press the Delete key on the keyboard.

To disable AutoComplete or clear your AutoComplete list entirely, follow these steps:

• Click the File menu and select Options.
• In the Outlook Options window click the Mail tab.
• Scroll down to Send messages. Uncheck the Use Auto-Complete List to suggest names when typing in the To, Cc, and Bcc lines box.

 To clear the AutoComplete list, simply click the Empty AutoComplete List button. You’ll see a confirmation window about clearing the AutoComplete list, click Yes.

Outlook AutoComplete should be completely disabled and any history that was in the auto-complete list should be completely erased (if you clicked the clear auto complete button).

Zimbra

In the Zimbra webmail client go to 'Preferences -> Contacts' and untick the setting 'Add new contacts to emailed contacts'. This will stop Zimbra automatically adding addresses you type in or reply to to your personal addressbook and hence it will not have then to suggest when you compose messages.

To see the various contacts it already has, either from the system addressbook, ones you have manually added, or ones it has stored because you emailed them, look under the 'Contacts' tab/link near the top. Within the emailed contacts you can highlight one or many addresses and click the delete button to clear them out if desired. You can also manually add addresses there that you do want it to suggest in future.

Thunderbird

In Thunderbird go to 'Preferences -> Composition -> Addressing'. Untick the option to 'automatically add outgoing email addresses to my...'.

To see any addresses already harvested click on the 'addressbook' button and look at the personal addressbook. From there you can choose to remove one or many addresses. You can also manually add addresses there that you do want it to suggest in future.