Mailbox Quota and Attachment Size in Office 365
Microsoft Office 365 mailboxes have a maximum storage quota as well as limitations on the size of individual messages that can be sent and received:
EUI users have been given a mailbox capacity of 50GB each in their mailbox. Once the quota is hit, mail cannot be sent nor received anymore.
To check your current quota, see How to Check the Size of Your Mailbox.
The message size limit is 25MB, including attachments. Messages larger than this limit will not be delivered, and the sender will receive a Non-Delivery Report (NDR).
Note that the encoding necessary to transform a file into an email attachment results in an enlargement of the original file size of up to 30%, depending on the file type.
The message size limit is a global setting that applies to all messages (that is, inbound, outbound, and internal).