Outlook's Default Attachment Size

Last reviewed on May 29, 2013

Attachment size warning We’re starting to see reports that begin something like this:

“We’re using a sendmail server. When we use Outlook 2010 and try to send attachments over 20MB we get an error: “The attachment size exceeds the allowable limit”. This works fine under Outlook 2003.”

The default limit is 20 MB (20480kb), which is about right for most ISPs. This limit eliminates the problem with stuck messages: If you try to send a message using SMTP, you get an NDR back into your Inbox telling you the message is too large. Outlook wastes a lot of time uploading the huge file that will never send and this prevents the sending of any other mail.

Exchange server accounts should use the message size set for the user account. Exchange 2010 users will see a mailtip above the To field instead of the warning dialog.

Mailtips warning

If the MaximumAttachmentSize is set higher on the client than the Exchange account is allowed to send, the message will not send. The Send and Receive Progress Dialog will show this error:

Task ‘alias@domain.com – Sending’ reported error (0×80040610) : ‘The message being sent exceeds the message size established for this user.’

If you can send larger attachments you can edit the registry to allow larger limits. While you can set it to 0 for unlimited message size, its better to set the limit to the size your ISP allows.

Registry editor

Open the registry editor and browse to:

Outlook 2013:


Outlook 2010:


Then add a new DWORD value: MaximumAttachmentSize

Unlimited message size uses a value of 0. For a specific message size (recommended) enter the value in Kb. You may want to round the value down to allow for overhead, so 50MB = 50000.

Users are reporting problems using the Send to mail recipient command in Windows Explorer when the default size is set to unlimited.

If your mail server limits you to smaller attachment sizes, you can set the key to prevent an NDR when you attempt to send messages that are too large.

Instant Fix

You can can change the dword value in code below if needed (its set for 30 MB) then copy and paste into notepad and save as “MaximumAttachmentSize.reg”. You’ll double click on the saved file to install it in your registry.

Windows Registry Editor Version 5.00


Download a ready-to-run registry file to set the maximum attachment size to 30 MB attachments

Outlook 2013Outlook 2010

Email Compression Tools

If you use tools such as WinZip’s Companion which compress large attachments automatically after you add them to the email message, you’ll want to set the maximum allowed size at least twice as large as allowed by your email service, or set it to 0 to disable completely. Outlook checks the attachment size as soon as you add the attachments, not before you send the message.

Written by

Diane Poremsky
A Microsoft Outlook Most Valuable Professional (MVP) since 1999, Diane is the author of several books, including Outlook 2013 Absolute Beginners Book. She also created video training CDs and online training classes for Microsoft Outlook. You can find her helping people online in Outlook Forums as well as in the Microsoft Answers and TechNet forums.

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