Email systems are not intended for archival storage. Employees are responsible for periodically purging email from their personal email accounts. You can save email attachments directly to OneDrive from Outlook on the Web. Once you have saved attachments to OneDrive, the original email can be deleted. Deleted email will be held in the Deleted items folder for 90 days before moving to the Recover deleted items folder for an additional 30 days. After that they will be removed from the system and not retrievable. The Recover deleted items folder is viewable by right-clicking the Deleted items folder on the web and windows versions of Outlook. For the Mac it may only be viewed from the web client.
Keeping an organized email account can help make purging unneeded emails as easy task. Finished a project? If you no longer need any information from that project, deleted the folder you used to organize those emails.