Outlook not allowing attachments larger than 1MB: How to fix?

The sending size limit setting of your mail server determines the attachment size restriction, or really message size limit, as it includes all of your attachments and the message itself. There may be issues regarding storage as you may only have less than 1MB of storage to spare, and for this reason, files cannot send if they are larger than 1MB. However, you can check storage by first signing into Outlook > Settings > View all Outlook settings > General > Storage; here on this screen, you can see the storage and permanently delete files that are no longer necessary, including emptying the bin. However, if that hasn’t resolved the issue, it’s an issue regarding the mailbox file size restrictions, which may be 1MB (1024kb). Follow the steps below to resolve this issue.

Step by step process – How to remove size restrictions of attachments from Outlook mailboxes?

  1. Sign into Office 365 using your credentials.
  2. Use the app launcher (nine dots) to navigate to the Admin dashboard.
  3. Under Admin centers click on Exchange, you may need to click “Show all” to access.
  4. In the Admin Exchange dashboard under “recipients” click on “Mailboxes”.
  5. Now click on “Set default message size restrictions”.
  6. Set a maximum size for messages that can be sent and received by the new mailboxes in your organization.

However, there are a few size constraints that will limit how much content you can send per Outlook email. You will need to set the maximum sizes for sent and received messages between 0 and 153600 KB. (KB or kilobyte is a much more refined scale of data measurement, so you can set the scale of measurement to precisely what you require).

Following the steps above should allow you to send large-scale content on Outlook mail as the main restrictions have now been lifted from the Outlook Exchange Mailboxes. You may need to visit each mailbox individually to see any specific restrictions related to the account in question.

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Increasing attachment sizes using regedit.exe.

The Windows Registry Editor (Regedit) is a graphical application included with the Windows operating system that allows authorized users to examine and modify the Windows registry.

In Outlook 2010, Outlook 2013, or Outlook 2016, you may increase the attachment size limit using software called regedit.exe. You can increase or reduce this restriction using a Registry change if your ISP (Internet service provider) allows for larger or smaller message size and you want Outlook to match it. If you’re connected to an Exchange server, you won’t need to do anything since Outlook will get the limit from the Exchange server automatically.

When sending an email attachment from Outlook, you may receive an error message stating that the attachment size is too large. Change Outlook’s default size to match the mail server’s default size when your mail server supports messages up to 25 MB and your attachment is slightly larger than the usual 20 MB limit. Follow the steps below to change those preferences.

  • In the windows desktop search bar type out “Run” and locate the application called “Run”.
  • Here on the dialogue box type out “regedit” then click on OK.
  • In the folder section locate “preferences” using these steps to navigate to the folder (Computer\HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook\Preferences).
  • Next find the REG_DWORD file called (MaximumAttachmentSize)
  • Ensure the base is set to (Decimal) and type out the value associated with your requirements.
  • Than click on “OK” 

Outlook File Size Limit:

Outlook does not transmit email messages with attachments greater than 20 MB by default; however, many mail servers accept 25 MB or more. Instruct Outlook to deliver messages greater than 20 MB if your email server supports larger attachments. If Outlook’s default message size is more extensive than your mail server allows, you can prevent receiving undeliverable mails.

In-depth step-by-step process – How to remove size restrictions from Outlook Mailboxes?

Breakdown of steps shown below:

Use your details associated with your Office account, for example, email, Phone even your associated skype account.

  • Use the app launcher to navigate to “Admin”.

The app launcher is illustrated by nine dots on the top left side of the display. You need to click on this to access the Admin portal.

  • Under the Admin center click on Exchange.

In this example “Exchange” was pinned so it was on the main menu panel on the left. If it isn’t for you, click on “Show all” and this will bring up the “Admin center,” here you want to find and click on “Exchange.”

  • In the Admin dashboard under “Recipients” click on “Mailboxes”.

Recipients is a dropdown collapsible menu option you need to click on the button to open the options and here click on “Mailboxes”. The Mailboxes are the recipients and other users who have a mail inbox associated with a company Outlook or mail account.

  • Now click on “Set default message size restrictions”.

This setting will allow you to change the restrictions for the entire Mailbox suite associated with the Exchange Outlook account. You may choose to have individual users still restricted to avoid spam mail or have only a few users not restricted if they have any importance to your organization.

  • Set a maximum size for messages that can be sent and received by the new mailboxes in your organization.

The final step is to edit the message size restrictions which will then set the mailbox restrictions from 1MB to over 150MB. You may need to visit the Windows Outlook registry to change the default settings to increase the limit if it is set to 1MB. You may also need to check to ensure any restrictions made to specific accounts are also lifted if you need it.


The points mentioned above are subjective to what the users’ preferences are and how the settings altered may not always result in restrictions being lifted. You may need to contact the Microsoft team to get more information on the specific reason why the restrictions are in place.

That’s it for this Blog thank you for taking time out to read our content, please feel free to email our team about how it went if you followed the steps or if you need more help with the questions we answered in this Blog.

Saajid Gangat

Saajid Gangat has been a researcher and content writer at Business Tech Planet since 2021. Saajid is a tech-savvy writer with expertise in web and graphic design and has extensive knowledge of Microsoft 365, Adobe, Shopify, WordPress, Wix, Squarespace, and more! You can connect with Saajid on Linkedin.

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