The message exceeds the maximum supported size., Cannot save changes made to an item to store.

Hi David, sometimes the max message size limits have been modified on an individual mailbox and do not match the default size limits for the tenant.  For example, the source admin might have increased the allowed sizes for a single employee who handles large contract documents. 

I recommend running the "Collect Statistics" task for mailboxes in ODM and selecting the option to "Collect statistics from target".  Shortly after this task completes, you will be able to download a Comparison Report from the task details.  Find the mailbox in this report and compare the Max Send Size Allowed and Max Receive Size Allowed for that specific user.  If target is smaller, then you can increase the target size allowance for that mailbox, or inform the user that large items will not be migrated.

Also some additional details on the limits is that EWS encapsulation can add an extra 30% to a migrated message.  See the chart below which shows 112MB as 33% under the 150MB message size allowed within Office 365.

112MB + 33% = 150MB

Users can send and receive messages up to 150 MB to each other, provided these messages never leave Microsoft datacenters, so these can be considered as internal mails.

Messages which are routed to outside of Microsoft datacenters are subjected to an additional 33% translation encoding expansion in size, such that in this case the maximum message size allowed is 112 MB.

OWA accounts for the possibility that your message may be subject to the 33% encoding increase and restricts the size of message you can send to 25% less than the configured setting. For example, if you customize your settings for a 100 MB maximum message size, you can send messages no larger than 75 MB.

Additional information ca be found here - https://learn.microsoft.com/en-us/office365/servicedescriptions/exchange-online-service-description/exchange-online-limits#message-limits

Doug Rain

Technical Product Specialist

Thanks Becky. I saw from my spreadsheet that one mailbox had different send and receive max sizes. I appreciate that.