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Essential has worked on some of the largest Public Folder migration projects in the world.  Here’s a few tips from our gurus:

A few years back you didn’t have an option to migrate your legacy public folders to Office 365 – in fact public folders on-premises were to be end-of-lifed.  SharePoint was initially tabled as an alternative, but this didn’t ‘wash’ with a lot of Microsoft customers because it didn’t offer the same functionality and was over-complicated.

Microsoft quickly changed its position (no doubt following uproar from lots of disgruntled customers) and now you can take advantage of modern public folders – a service that seems to be hanging together reasonably well and growing bigger in capacity all the time.  It’s now 100TB in total – it started out at 2.5 TB and then 50TB so it’s always worth checking here Exchange Online limits – Service Descriptions | Microsoft Docs!

As you might imagine, there are some caveats, clean-ups and other considerations that come into play if you want to make the move.

But first off, it’s worth getting a bit of background on the modern public folder construct:

The Modern Public Folder service is very different from the Public Folder database architecture you’ll already be familiar with.  It basically uses regular mailboxes that are automatically linked together and load-balanced (for Office 365) as your Public Folders grow in size.  Being regular mailboxes they also benefit from being part of data availability groups (DAGs) instead of having to undergo painful public folder replication.

Here’s how the modern public folder to Office 365 architecture works:

  • You kick off with a single, Primary Public Folder (PF) mailbox (which can grow up to 100GB in size)
  • Office 365 detects when a PF mailbox is approaching the 100GB limit and uses an auto-split feature that creates a linked Secondary PF ‘overspill’ mailbox.
  • As the next mailbox fills up, another PF mailbox is added and content is automatically re-balanced across all the mailboxes.
  • This expansion continues until you hit an overall limit (at the time of the last update to this article it is 1,000 public folder mailboxes and 100TB in a single Microsoft 365 tenancy).
    See this page for the latest info:
  • A PF hierarchy is maintained alongside the PF contents in the Primary mailbox.
  • This hierarchy is updated to reflect the new location of items as new PF mailboxes are added and as content gets ‘re-balanced’ across the available mailboxes.
  • Read-only copies of the PF hierarchy are also stored in each of the Secondary PF mailboxes and these are kept in sync with the Primary using Incremental Change Synchronisation (ICS).

The key thing to note that is that as far as users are concerned, although the Public Folder to Office 365 mail comprise multiple, ‘lashed together’ mailboxes, they can be viewed and navigated as a single, logical entity.

This is a really great PowerPoint by MVP Peter Schmidt that describes the whole thing in more detail:

See also this Microsoft document for details:

Planning Your Migration

Can you migrate?

If you’ve already upgraded to Modern Public Folders on-premises (i.e. you’re using Exchange 2013 or above), Microsoft Office 365 does not currently offer a ‘native’ migration solution.

At the time of writing you will need to look to a third-party migration solution to help out.  If you don’t want to go down that route, the other option is to keep your PFs on-premises and access them from the cloud until Microsoft delivers a solution.

If you are using ‘old school’ PFs (aka legacy PFs) hosted on Exchange 2010 SP3 RU8 or later or Exchange 2007 SP3 RU15, Microsoft has a migration solution using batch migration scripts as described in this article:

You’ll need to run around 11 separate scripts in total (including a final synchronisation and switch – yes – it’s using MRS) which means it can be quite complicated to use.

Using a third-party tools can simplify the process. The tool from Binary Tree (now Quest) is interesting as it performs a two-way PF synchronisation between Exchange on-premises and Office 365.  This has the benefit that all users are able to continue to access up-to-date PF content regardless of where they are in the migration process – on-premises or in the Cloud.  You can also elect when you migrate yours PFs, as otherwise you would typically wait until you have migrated all your mailboxes into the Cloud.

There’s another neat tool that we offer that you might want to check out too.

If you’ve been archiving public folders in the past, for example, using Enterprise Vault, we can help you migrate archived public folders, and indeed regular PFs, to Office 365.

At a push you can also use PST files as a mechanism for uploading on-premises PFs into Office 365, but you need to know what you’re doing when it comes to splitting your PFs into ‘mailbox chunks’ (see below).

Do an Inventory and Have a Clean Up

Some of our customers store vital customer records in PFs.  They also have a lot of rubbish in them and migration is a great opportunity to do a sort out.

Start by doing an inventory of your PFs at a ‘high-level’, and get statistics such as size, item count, owners, permissions and last accessed dates.

In order to make solid and defensible decisions around whether content can be deleted prior to migration you’ll need to do a LOT of deeper digging, however gathering initial meta-data can give you some excellent pointers.  For example:

  • Removing empty and duplicate folders can be a quick fix.
  • Orphaned folders with an old last accessed date are a very obvious candidates for a clean up.
  • Knowing the owner of a PF (assuming it’s not ‘Administrator’) can help signpost who you need to contact in order to see if content can be disposed of.

As ever with records disposition decisions, seek to get the relevant data custodians to call the shots – don’t go it alone!

Bear in mind that a potential downside to deleting or excluding older/stale contents from your migration is that you could create an eDiscovery headache later. For example, an HR dispute may refer back to employment terms and conditions, pension fund arrangements, etc, that were published decades ago.

Analyse Your PFs for Potential Glitches

Given the inherent differences between the architecture of old PFs and Modern PFs, you’ll need to spend some time eliminating things that will upset the migration process. For example:

  • Check for stale permissions
  • Check there are no orphaned PF mail objects or duplicate PF objects in Active Directory
  • Check PF names – syntax errors in your legacy PF naming convention can cause problems. For example:
    • If the name of a PF contains a backslash () it will end up in the parent PF when migration occurs.
    • Trailing whitespaces within Mail enabled PFs and commas in the Alias field will also create synchronisation problems.
  • Check all mail enabled folders to see that they have the right proxy address.
  • If you have any forms, these need to be exported and re-imported into Office 365
  • If users have PF ‘favourites’, they will need to document these before you cut over, as they will disappear

Chunk Up Your Legacy Folders to Slot Nicely into the New Separate Mailboxes Model

As we said earlier in this article, Office 365 performs an auto-split and load-balancing function as PFs approach 100GB in size, but this process can take up to two weeks to complete.  This is not usually a problem when you are populating a PF during ‘normal use’, but when you’re in a midst of a wholesale migration, you’ll be chucking data into Office 365 PFs at a rate of knots, and Office 365 can’t recalibrate itself fast enough.

Common to all migration approaches, therefore, is the need to take the Office 365 PF size restriction of 100GB per mailbox into consideration and effectively run scripts to ‘chunk up’ your PFs into separate PFs that are less than 100GB in size before you start your move.  We suggest you check that your ‘chunks’ are split according to logical subfolders.

Don’t overlook that fact that some of the items in PFs may be archived, as this will not only impact how you do your migration, it will also impact your sizing analysis (as shortcuts to archived items can be a fraction of the actual item size).   Check the message class to do this – e.g. IPM.NOTE.EnterpriseVault.Shortcut

There are many other considerations to take on board to ensure the best outcome post-move, such ensuring optimum retrieval times by putting PFs in a geographic location that’s near to users that will be accessing it.  Ensuring the number of people accessing PFs is kept below 2,000 per mailbox is also recommended.

Post-move you’ll need to do lots of checking and you might also need to re-mail-enable mail-enabled PFs post migration as this attribute might not get migrated.

You can find other considerations here:

Essential can help you with reviewing your public folders migration to Office 365, and can provide insights that include:

  • Storage Trending
  • Public Folders by Access Time (Tree View + List View)
  • Public Folders by Size (Tree View + List View)
  • Top 10 largest folders
  • Empty PFs
  • Top 25 Public Folder owners
  • Public Folders by Last Post

Let us migrate your Public Folders to Microsoft 365 (or elsewhere!)

We can simplify your Public Folder & Public Folder Archive migrations – or help you migrate to alternative platforms like Azure  Get in touch to discuss your options.

Along with PSTs, Public Folders are usually the last data repository to be dismantled when you’re migrating to Office 365.  They’re typically big and full of static content, much of which is ‘ROT’ (redundant, outdated and trivial information).

Many public folders, however, contain active, business-critical information, especially where mail-enabled public folders have become enmeshed in business processes, such as the transport company we work with that uses public folders to manage and share all communications relating to all its shipments.

This means they can’t leave them behind. Apart from upsetting users and impacting efficiency, you might compromise future legal discovery situations.

For a while, Microsoft didn’t have the equivalent to public folders in Office 365.  They eventually capitulated with the provision of a ‘modern public folder service’ in Exchange 2013, – even so, large ‘legacy’ public folders are proving to be tricky to move with any degree of ease and success.

Industry experts signposted Groups as the obvious alternative to public folders – and bet development dollars on tools to help move your public folders to Groups, but these never really got traction.

We are knee-deep in the migration of some of the UK’s largest public folder estates and discovering that even the best tools on the market are stopping short of providing a silver bullet.  

In fact, many vendors, such as Quadrotech and MessageOps, have simply thrown in the towel when it comes to providing a fault-free public folder migration tool.

There’s no wonder!

Here’s a snapshot of just 6 obstacles we’ve been navigating recently in migrating public folders:

Large public folders

Part of the challenge of moving on-premises public folders to modern public folders is that you must partition them up into sub 100GB* sized chunks and distribute them in a way that ensures optimum accessibility and performance for users.

There’s a whole bunch of limitations you need to consider to ensure you stay within Microsoft’s recommendations.  This includes a limit of 10,000 sub-folders and 1 million items per folder, the number of concurrent users accessing a public folder and many more – check out this article from Microsoft for more information.

Archived public folders

Archived public folders content complicates the sizing process. Emails that look like a few KBs in size when you run an initial analysis could be linked to much bigger emails and attachments (and cumulatively ‘blow’ the 100GB limit as you try to migrate).  Taking a two-step approach and first re-hydrating archived content into Exchange is a good approach for knowing exactly what you’re dealing with, but it means you must be able to cope with an interim storage challenge and a protracted migration timeline.

Public folders you *think* haven’t been archived

Even if you feel confident that certain Public folders haven’t been archived, it’s highly likely they contain archived items that you don’t know about.  This is because users might have dragged short-cutted (archived) items into them. To tackle this, you will first need to map all the relevant retrieval paths, which could be from several legacy archives, and then rehydrate the original items into Exchange prior to the migration.

Too many options as to where to move your public folders

Migrating legacy public folders to modern public folders in Office 365 is just one option open to you.  Navigating your way through the alternatives (e.g. SharePoint, Groups, shared mailboxes, Teams – even non-Office 365 platforms) and understanding their limitations and benefits is mind-boggling, e.g.:

  • Groups – Good support for public folders collaboration features and better mobile support, but have a flat structure and no permissions granularity;
  • SharePoint – great versioning support, check-in/check-out, but not mail-enabled.
  • Shared Mailboxes – Good hierarchy support and granular permissions, but not possible to mail-enable specific folders and customize the mailbox view to exclude default folders such as calendar, contacts, drafts, deleted Items, sent items, etc.

You also have the option to archive off static public folders content to other locations such as a cloud-based archive, in which case you need to consider things like end-user access, access control, eDiscovery and so on.

Migrating public folder permissions

When you relocate public folders content, mapping access rights correctly is vital – especially with concerns like the GDPR to contend with.  This step is relatively straightforward if you’re migrating to public folders online, but not if you’re moving to a non-Office 365 platform, such as an Azure-based archive.

Even though you can extract email recipient information as the basis of governing access, coping with very large public folders combined with the additional overhead of expanding the members of distribution lists, creates a whole new challenge.   You might also take your migration as an opportunity to tidy up and streamline permissions.

Tackling invalid characters in public folders

Trailing spaces, the wrong sort or dash (that longer dash they use a lot in the US), back and forward slash – these all need to be removed from your legacy folder names prior to migration. Otherwise, the default may to move their contents to the parent folder and blow the storage limit.  In actual fact, this is the easiest bit to tackle.

Final thoughts

Each project has its unique technical challenges.  It’s clear that the key to working out your best migration strategy is to perform in-depth analysis before you start.

Check out our free public folder analysis tool.

It’s unlikely you’ll find a single silver bullet for your specific needs, but following your analysis we’ll be happy to make recommendations and share with you what we’ve learned so far in terms of the best tools and techniques around to help with your move.

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