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Message Limits
The following limits are applied to every email message.
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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).
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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.
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The default maximum message size for Office 365 mailboxes is 25 MB.
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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.