Guided by principles in GDPR you may wish to delete mail older than 3 months from your inbox. This should be a principle for all office workers in both private and public organizations: Mail and electronic content not journalised should be deleted within 3 months.
Outlook: Choose ‘Account Settings’ in File window. Under ‘Mail’ highlight account and choose ‘Edit’. Set time period for storage on Exchange server to 3 months. Repeat as necessary for all accounts. Under each account in Mail window: Right click inbox and chose ‘Properties’ and click the option to delete all offline content. Repeat as necessary for sent mail and other accounts.