How to Fix Outlook Performance Issues with Large Mailboxes?

Written By Ashwani Tiwari
Anuraag Singh
Approved By Anuraag Singh
Published On April 8th, 2024
Reading Time 7 Minutes Reading

Are you facing Outlook performance issues with large mailboxes or is your Outlook has too many items and folders in Cached Mode? If yes, then the article is for you. Learn here, what to do and how to do when facing Office 365 Outlook performance issues.

Microsoft Outlook is the most popular email client used by the business as well as personal users. It is one of the oldest email clients with lots of versions like: Outlook 2000, 2002, 2003, 2007, 2010, 2013, 2016, 2019 and many other previous editions. As it is so much in use among the users so it contains lots of important data. But the problem starts arising when there is a large number of items in a single folder. The most common problems that cause Outlook performance issues with large mailboxes are:

  • If all default folders like Inbox, Sent Items, Calendars, Tasks, and Deleted Items folder of Outlook contains a large number of items.
  • When a user tries to perform some particular action using Outlook data file (PST) or Cached Exchange Mode.

So, if the Outlook mailbox contains a large number of items in its mailbox, then the size of Outlook mailbox will increase unnecessarily large. This will lead to starting creating issues like performance problems, efficiency problems in the Outlook account. Therefore, to avoid all Office 365 Outlook performance issues and to improve Outlook 2019, 016, 2013, 2010 performance and efficiency, there are some tricks available, that anyone can follow easily.

Reasons: Outlook Performance Issues With Large Mailboxes

Yes, it is true that Outlook data files and mailbox folders have some predefined size limit. However, there are plenty of reasons behind Office 365 Outlook performance issues:

  • In Outlook 2002 and below versions, account starts degrading after 2 GB usage of PST data files.
  • If a user is using Outlook 2007 or above versions, then the performance issues will starts after 12 GB of usage.
  • In Outlook 2003 it starts degrading after 4 GB of usage.
  • If the folder exceeds the number of items per folder limit or mailbox contains a large number of emails, then also it starts creating issues.
  • In Outlook 2007, a user is allowed to have total 50,000 items in each folder and 500 folders.
  • In Outlook 2010, 2013, 2016, 2019: 100,000 items per folder and 500 folders in total are allowed.

Quick Solution to Resolve Office 365 Outlook Performance Issues

One of the best solution to fix Outlook performance issues with large mailboxes is by removing PST file corruption via recovery software and improve your MS Outlook performance effectively.

In some cases where Outlook data file exceeds its predefined size limit, then it becomes corrupted. Thus, to repair corrupt PST file, users are suggested to take help of a third-party tool. The SysTools Outlook PST File Recovery software is one such application that removes all type of corruption from PST file and brings it back in the healthy working state. Moreover, it provides split PST option also to break large PST file into multiple parts. While using this application, there will be no loss of data. It is capable to retrieve lost data within the PST file.

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Large size mailbox can create lots of trouble in the Outlook. And it degrades the performance of Outlook 2019 / 2016 / 2013 / 2010 / 2007, etc. Sometimes it leads to corruption in PST file, which is hard to fix without any expert help. Thus, to overcome all such issues and fix Outlook performance issues with large mailboxes, try the following manual methods.

How to Fix Outlook Mailbox Performance Issue Manually?

If the Outlook is working really slow and at an unstable stage, then it might be a serious problem for your productivity and even increase the stress level. It is possible to improve the performance, you need to reduce Outlook PST file size . A user just need to follow the simple manual tricks discussed below to instantly resolve Office 365 Outlook performance issues when there are too many folders and items in Cached Exchange Mode Outlook files: :

#1 Method: Keep PST File at Reasonable Size

In Outlook 2003 and 2007, a user should create a PST file of new style without any 2 GB file size limitation. But, to have consistent performance, a user is advised to keep this new PST file small in size i.e., do not let them grow above 1 GB. Now, to stop PST file from growing large, by following the tips discussed below Outlook performance issues with large mailboxes:

  • Create multiple PST files and organize them properly.
  • Try to extract attachments from PST file and store it somewhere else.
  • Archive complete mailbox of Outlook time to time.

 #2 Method: Store Less Items in Mailbox Default Folders

In order to improve Outlook performance issues with large mailboxes Always try to have less than 5,000 items in the Inbox and Sent Items folders of Outlook mailbox. It is because whenever Outlook starts up, Inbox and Sent Items are the two folders with which it synchronizes very first.

And, if it contains a large number of emails in it, the Outlook will struggle during startup. Thus, the solution creates new folders and keeps some emails in those folders. Moreover, a user can create and apply some mail rules filter also to receive messages in that folder automatically. In order to know the total number of items in the folder, simply click on it and see at the bottom left corner of the Outlook and fix Office 365 Outlook performance issues.

#3 Method: Compact PST files Size

When a user is done with the splitting PST file and removing attachments from PST file to Outlook performance issues with large mailboxes, its time to compact it. Shrinking of PST file will compress the size of PST file and enhance the performance of Outlook email client 2019, 2016, 2013, 2010, 2007, etc. However, for the Exchange users, you should definitely to compact OST file also for better results.

All these manual tips and tricks are useful until all Outlook data file i.e., PST file is in a healthy state. If the PST file gets corrupt due to large size mailbox, then it becomes difficult for the users to optimize it without removing corruption.

#4 Methood: Try to Use Cached Exchange Mode

A user should use cached Exchange mode as it will enhance the performance of Outlook 2019, 2016, 2013, 2010 and all below editions. Usually, people run Outlook in Online Mode and in that mode it reads directly from the server. This can cause Outlook performance issues with large mailboxes and slow internet.

However, in the cached mode, the Outlook will maintain a replica of the mailbox on the computer. And, it synchronizes with Exchange Server. Yes, this mode takes lots of time to sync for the first time. But after that, its performance gets improved automatically. Moreover, in this mode, a user can easily access mailbox when there is no internet connection.

The above methods provide a proper solution for a user query: How to fix Outlook performance issues with large mailbox PST file? If nothing works, then go with the software suggested above. It may help to resolve your Outlook performance issues effortlessly without any data loss or risk. This can serve as the best solution for any versions like Outlook 2019, 2016, 2013, 2010, 2007, 2003, etc.


Today, we resolved Outlook performance issues with large mailboxes through 5 different methods. If you are an Outlook user and have too many items and folders in your Outlook mailboxes, then the above article might have definitely helped you. However, to avoid Office 365 Outlook performance issues, you should immediately get rid of corrupted files and folders, store less items in Outlook folders, keep the minimum size of PST files, and adjust sync settings.


By Ashwani Tiwari

Being a Chief Technical Analyst, I am aware of the technicalities faced by the user while working with multiple technologies. So, through my blogs and articles, I love to help all the users who face various challenges while dealing with technology.