This list is not permanent and addresses you want to keep should be added to your Contacts folder, either manually with a right click on the address or using one of the automated tools below. When using Outlook, addresses you reply to are added to your autocomplete cache. If you are using Outlook 2010, Suggested Contacts can be disabled in File, Options, Contacts but the folder cannot be deleted until you upgrade to a newer version. The Suggested Contacts feature was removed from Outlook 2013 and up, as many people found it confusing and few people wanted the extra folder cluttering their list. Because the folder is separate from your default contacts folder, it avoids many of the problems associated with saving all addresses you send mail to. Microsoft Outlook 2010 adds addresses you reply to, to the autocomplete list and if a contact does not already exist, it creates a contact in the Suggested Contacts folder used specifically for contacts it automatically creates. You can use VBA to create Contacts from messages in the selected folder or automatically add recipients from outgoing messages to Contacts.

You can always add any address by right-clicking on it and choosing Add to Outlook Contacts.

I personally think that adding addresses automatically is a bad idea, unless you're very restrictive with the rules that trigger the addition. You are responsible for making sure that the addresses you add automatically are kept up to date.

How to import contacts into outlook web