Preparing your Exchange migration – Checking your size and item count

You’d be surprised about how people use their email as a catch all for everything. In the last 17 years I’ve seen some strange things such as people using their deleted items as their filing area, send items NEVER being attended to and a to z filing systems where most folders are empty…just to name a few. If you are planning your Office 365/Exchange migration and it is the backbone of your migration, you will want everything to go smoothly and thus you will want this piece to show case the fact that moving to Office 365 was a good idea. In a few blog entries I am going to talk about a few pitfalls to watch out for.

Today’s pitfall is knowing that Exchange Online has some limitations that you need to be aware of. If you are working with a company that has been around for awhile, you will have the possible 10-20 year employees that don’t delete anything, and depending on their email activity over the years, they might have a lot of emails and not knowing ahead of time can cause your project some issues or some mailboxes to fail their migration or in the least take a REALLY long time to migrate.

Exchange Online Limitations

There is a great site published by Microsoft that I will publish at the bottom of this article but it’s important to know some of the limitations of Exchange Online/Office 365. I believe that Microsoft has tried hard to even accommodate the edge cases, but there will likely be at least one problem mailbox. The first limitation we want to look at is sheer size of the mailbox and there are two metrics that we will want to consider here (Size of Mailbox and Size of some of the messages). In most cases the max size that Microsoft will support in Office 365 is 50 GB, so make sure that none of your mailboxes exceed 50 GB or plan to keep that mailbox on premise, it might be valuable to consider keeping some of your larger mailboxes on premise as well if they are creeping close to the 50 GB mark. The second and lesser piece to consider is individual messages, this one is a little bit more finicky but shouldn’t be a problem if you’ve managed to keep your attachment sizes either default or manageable in your on premise environment.

Microsoft mailbox size max

Using the following table and all of the tables on the Exchange Online Limitations site, you will notice that you will need to know which version of Office 365 you will be using as the limitations are slightly different for each, in this case most are the same and the hard limit to be aware of is 50 GB.

Microsoft Message Size limits

Another important consideration that I’ve never seen an isssue about, but I really want to draw your attention to the numbers for “Message size limit – Outlook for Mac”. It’s 35 MB, so it’s important to set expectations in your user community that increasing allowable attachment size limitations may cause some issues with Outlook for Mac.

The other number that I’d like to bring to attention is the total message limitations.

Microsoft Item limit

1 million might seem like a large number and to be honest I’ve only come across it once in my IT career. At the same time I’ve come across this once in my IT career and it happens to be with an Office 365 migration. Not knowing this could be the difference between the project floundering and it going completely smoothly.

The full list of Exchange Online Limitations can be found hereĀ

The PowerShell help

PowerShell will be your best friend in this endeavor. You will want to dump a full listing mailbox, mailbox size and item total to an excel spreadsheet. This will help you know what you have, and work through the planning steps to create a successful migration plan.

The command to get a spreadsheet with mailbox name, total items and total size will look something like this

Get-MailboxStatistics -Server mailserver1 | select DisplayName, ItemCount, TotalItemSize | export-csv -path C:\Users\%username%\Desktop\exchangeexport.csv

I can’t say it enough, doing this leg work will help with a successful migration and having to explain the limitations later on. The best projects set expectations and limitations and are proactive to accommodate those findings.


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