You’ve decided to upgrade to Exchange 2013. You’ve heard that Exchange 2013 causes “Mailbox bloat.” You’ve got tons of different mailbox quotas, and there does not seem to be any real rhyme or reason as to why they are applied.

Fear not citizen! I am here to explain an easy and logical way to determine how to set mailbox size limits for your new Exchange 2013 environment.

How much mailbox bloat am I going to see when I migrate to Exchange 2013?

We consultants do love a good loaded question.

Yes, you will see the calculated size of your mailboxes go higher after they are migrated to Exchange 2013. You can expect that mailboxes will grow 25 to 40 percent larger after they are migrated from Exchange 2010.

However, no, you will not see any “mailbox bloat.” Your mailbox databases are going to be exactly the same size as they are now on Exchange 2010.

How is that possible? 40% larger mailboxes, but no growth in mailbox databases sounds crazy. The answer is really very simple. Exchange 2013 does a better job of attributing items in the database to the users with whom they are associated.

For example, in Exchange 2010 when you clear out your deleted items folder you’ll see your mailbox size shrink. However, those items may not really be gone because you can still recover them with deleted item retention. Exchange 2013 still sees those items as part of your mailbox until they are really gone.

My mailbox on Exchange 2010 is 5 GB, so I should make it 7 GB when I move it to 2013?

As a consultant, my primary obligation is to never answer the question I am asked. Whenever possible I am required by law to start my answer with “it depends…”

You’ll be moving your mailboxes to a new platform that has new features and new ways of doing things. In addition to that, I suspect that most current mailbox size limits were determined by a liberal application of wild guesses.

Steps for setting up Mailbox size limits

Why don’t we start fresh and do this thing right?

Step 1: Determining the Mailbox quota

The first step in determining an appropriate mailbox quota is identifying a messaging profile. A message profile is the combination of how many messages are sent and received per day, and the size of those messages. I always recommend that before anyone starts an Exchange migration, they take the time to determine messaging profiles for all their users, and then create logical groupings of those users.

Step 2: Determining the email retention period

Once you know how much email your users send and receive, the next step is to determine how long each grouping of users needs to retain email messages. Maybe the accounting department needs to keep email for 2 years, and the C-level executives need to keep email for 7. Whatever those retention periods are determined to be, we multiply the number of email messages sent and received per day by the average size of those emails by the amount of time emails need to be retained.

Now we have good information to use to size our mailboxes, and the rest of our Exchange design process can proceed from here.