Advice on Mailbox Management & Data Retention

swgit used Ask the Experts™

I'd like to know what your thoughts are or industry best practices in regard to leaving gigs and gigs of emails on Office 365 vs. archiving emails into pst files onto local/on premise archives/backup servers.  What are the pros and cons? Thanks.
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Tom CieslikIT Engineer
Distinguished Expert 2017
I think that it is always good to have a backup. You'll never know what can happen.
Also if in the future you'll decide to move away from O365  and move over to your own Exchange or use some other 3rd party email service then it's better to have copy of all your email data.

If you have a space in your location then don;t hesitate to do it.
This is my opinion.
Most Valuable Expert 2015
Distinguished Expert 2018
O365 includes a lot of storage for a reason - the recommended approach is to keep everything in one place. Not only as a user you can access it from anywhere, but it's centralized for eDiscovery and compliance needs. They are finally delivering on the "Unlimited archiving" part as well, though that solution leaves a lot to be desired.

In any case, most people prefer to keep everything in O365, especially when combined with litigation hold. There's also the Inactive mailboxes functionality that allows you to store data for leavers at no cost. The only valid reason for having external backup is if you want to be able to perform a point-in-time type of backup, but we can argue how appropriate the available 3rd party solutions are for that. Well, and for some specific compliance requirements.

And no PSTs! PSTs are bad and you should feel bad for even considering using them! :)
Chief of Staff
Hi there.

Ok, there are two parts to this question.

First, is it ok to leave loads of data in Office 365?

Second, is there merit in using a 3rd party tool to archive/backup locally.

On the first question, yes, it is ok to leave tons of data in Office 365 and Exchange Online. What you need to do though is ensure that you understand why you are keeping data.

Look at what regulation applies to your industry/country and find out if there are any specific rules you need to adhere to.

Once you understand that, look to how you need to structure your policies for retention. For example, do you agree to keep everything for ever or do you automatically remove data over a certain age limit.

Then you need to make sure that you apply litigation hold to your user mailboxes because you may be legally obliged to produce emails that your users may have deleted.

Now the Office 365 retention and policy mechanisms are good enough for the vast majority of industries but for some who have very specific or are extremely risk adverse use cases, you can look to 3rd party archives.

In this day and age you would never ever backup to PST as this is dangerous, unmanageable and will create a major problem for your or one of your successors in years to come.
Pst's are not easily indexed, searchable and can become corrupt, lost, duplicated and more. Just don't do it.

Rather look to one of many cloud based archive vendors if your business cannot trust Microsoft to hold your data for a long period. Companies like Mimecast, Proofpoint and many more have proven track records and will give you all the additional archive functions you may feel O365 is lacking.

As for backups....

If you ever decide to leave Office 365, you will have to migrate your user mailboxes to the new location. Having data in Office 365 does not make this any harder.

My company ( has just extracted and migrated 18TB of mailbox data from Office 365 in under two weeks, so don't think that your data is locked in place, it isn't
swgitIT Professional


Thanks, all.  These are very helpful!!

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