Late yesterday Microsoft announced that they are now supporting mail message up to 150MB in size. Previously the message size limit was listed at 25MB (but actually set at 35MB) and could not be changed. With the change to allowing mail messages up to 150MB, Microsoft is also allowing the company’s IT department the ability to configure the mail message size limit from 1MB to 150MB. If you don’t need or don’t want to change the message size limit, that is just fine, it will remain at the 25MB limit. See below on how to make changes to the message size limits in Office 365.

To change the default message size limit for all newly created mailboxes, you can do this via the Exchange Admin Center (EAC) or Remote PowerShell.

In the EAC go to the recipients\mailboxes section and client on the 3 dots (…) and select the “Set default message size restrictions”

As you can see above the default size limits are set for sent and received mailboxes. I went ahead and changed the defaults for my tenant to 50MB for both send and receive.

I could have also done this via PowerShell, let me show you

The above command changed my size limits to 75MB for both send and receive.

Now that I have updated the limits for all new mailboxes, I need to modify the limits for the existing mailboxes in my tenant.

While you can update existing mailboxes in the EAC, most organizations with a large number of mailboxes would want to use Remote PowerShell to make the changes. (Use the link to the Microsoft blog above to see the EAC method for changing the size limits on existing mailboxes)

The above command changed the size limits for all of the mailboxes to 75MB for both send and receive.

Above I decided just to set my mailbox to 100MB for both send and receive size limits.

I recommend you read the blog post from Microsoft, has some good information on details about changing the size limits.