The following limits are applied to every email message.
Message size limit: Message size limits are necessary to prevent large messages from blocking delivery of other messages and affecting service performance for all users. These limits include attachments, and apply organization-wide to all messages (inbound, outbound, and internal). Messages larger than this limit will not be delivered, and the sender will receive a non-delivery report (NDR).
Some email clients may have lower message size limits or may limit the size of an individual file attachment to a value that is less than the Exchange Online message size limit.
The default maximum message size for Office 365 mailboxes is 25 MB.
Exchange 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.