Are there any details on how long emails are retained on outlook.com? Is there a way to recover emails that have been removed by outlook.com?

Ever since I have migrated from the Hotmail site to outlook.com I have been losing older emails (4 to 5 years old)?  Is there a way for the outlook to recover emails they have removed?  Is there a retention policy?  What is the recommended back up process so that I do not lose emails?  I have kept my email on email servers for back up purposes over the years.  If this is not the preferred method I need to know what is.  Thank you