WOOF! Newsletter

December 07, 2017

Fight Mailbox Bloat: Why Outlook Slows Down Over Time

Outlook can slow down if you have an extremely large Exchange mailbox. Some users have tens or even hundreds of thousands of business emails from many years of work. Without an archiving solution, those accounts may experience performance issues due to “Mailbox Bloat”!

How to Handle Extremely Large Exchange Mailboxes

One day you notice your email isn’t responding like it used to. You click a folder and it takes a moment to open. Or you start typing an email and find you’re waiting to see the text on the screen. Maybe Outlook starts crashing on you randomly. Or search isn’t finding the emails you know are there.  What’s happening?  

One of the most likely causes? Your Exchange mailbox is too big.  You have Mailbox Bloat.

Let’s look at Mailbox Bloat—what it is, how it happens, and what you can do to avoid it.

Anatomy of an Exchange Mailbox:  How Your Outlook Email is Stored

To explain where & how your emails are stored, we need to take a look at email architecture.

Exchange Server holds your email account in a database. Outlook talks to that database to get your emails. To do this, Outlook operates in one of two modes: Cached Mode or Online Mode.

  1. In Cached Mode, Outlook keeps a local copy of your mailbox’s emails, tasks, etc. in files on your hard drive. These are typically .OST or .PST files.
  2. In Online Mode, Outlook doesn’t keep a local copy of the mailbox. Your emails ‘live’ on the Exchange Server, but you still work with them through Outlook.

The problem is that those .OST/.PST files don’t have an “upper limit” on how big they can get. If left unmanaged, they can grow to 5 GB, 10 GB, 20…

How Big is TOO BIG?

When you use Outlook, the following .OST size guidelines generally apply: 

  • Up to 5 gigabytes (GB):  OK on most hardware.

  • Between 5 and 10 GB: If you have a fast hard drive/SSD and lots of RAM, you should have no problem. If your computer is older, you’ll start experiencing little pauses while you work in Outlook (reading emails, moving them to folders, deleting).
  • More than 10 GB:  When the .OST file reaches this size, most users start seeing short pauses in Outlook.

  • 25 GB or Larger:  When the .OST file is this big, you’re in for problems. Outlook will run slowly almost all the time. It may crash or freeze at random.

How “Mailbox Bloat” Happens

Chances are you save all your emails. Never know when you might need them, right?

One email doesn’t take up a lot of space, but they build up over time.  According to Lifewire, 269 billion emails are sent every day!  (Sadly, most are spam.)

The more emails you save, the bigger your .OST/.PST files get. All the folders in your Outlook? Part of the .OST/.PST file too.

Attachments make it worse.  It’s extremely common to send files via email attachments. But if everyone sends files around by email, your mailboxes will bloat like crazy! (Users should have a file sharing option to store & share their files without clogging up email.)

The Big Problem: Performance Drags

Performance becomes a big issue if Exchange mailboxes grow larger than 10 GB.  This can and does happen often! 

It doesn’t just affect you either. An overworked Exchange Server with lots of bloated mailboxes will slow down too. Everyone’s email gets slower. You start having send failures. Which quickly becomes a serious office-wide problem!

Mailbox Bloat or Old Hardware? Use Windows Search to Tell

You might think, “Oh, this computer’s old, that’s why I’m having trouble. I just need a new computer.”

While that might be true, there’s one way to tell if Outlook is slow due to Mailbox Bloat, or if it’s just your old computer. It’s Window’s Search tool.

The Windows Search tool indexes all the files and apps on your computer, including the data within Outlook’s .OST/.PST files. However, it can only index Outlook data while Outlook is running.

If you have a huge .OST file, it will take Windows hours to index it. And it’s doing so while you’re trying to use Outlook!

Solution: Leave Outlook open overnight, and see how well it performs in the morning. If you don’t experience the same slowness as yesterday, then you have a big .OST file (and an oncoming case of Mailbox Bloat). If you’re still seeing bad performance, your computer may indeed need upgrading.

How to Fight the Mailbox Bloat

Okay, we’re sure the Outlook performance problems are due to Mailbox Bloat. What do we do about it?

  1. Delete some old emails!  Simple and effective. (Never delete business-critical emails though.)

  2. Educate users with the biggest mailboxes. They should know they can delete old non-critical emails, and should do so regularly.  If they need to keep all their emails (and some do), an easy solution is to setup Outlook folders for certain date ranges, and move their emails into those folders (1ST QTR 2017, for instance).  That relieves space in their Inbox, which should improve Inbox performance, but still gives them easy access to their old email.

  3. Activate AutoArchive.  AutoArchive helps you reduce the size of your mailbox, by deleting or moving old items into a separate data file on your computer. The items (emails, tasks, etc.) are still there, they just aren’t included in Exchange’s daily activity. Saves you space, and less bandwidth/time used.

  4. Switch to SSD drives. This doesn’t cut back on Mailbox Bloat, but it does make larger mailboxes easier to access. SSDs are much faster than hard drives.

  5. Implement a long-term archiving solution. This allows you to archive old emails after a period of time (e.g., one year) and delete old emails from your mailbox.

Mailbox Bloat Isn’t Inevitable! Keep Your Outlook Performing Well Over Time

Mailbox Bloat creeps up on us over time. When it gets bad, it’s hard to fix. However, if we all make a habit of keeping our email folders tidy, the bloat stays away. Outlook keeps running fast, nobody has send failures, and everyone’s happy with email.


Do you save lots of email (and might have experienced Outlook performance issues)?  Email us at woof@planetmagpie.com and tell us what you think.


Robert Douglas, IT Consulting Team Lead